The Matchbox kit of the Hanomag 251 is of the B version of the vehicle. If you want to make a C version, as you might if you were going to make a later variant of the vehicle such as a 251/9 Stummel, then you may find the instructions below useful. They do not address the internal differences, and confine themselves to the external appearance of the vehicle. C versions were more common than B versions, later in the war, and so for a later armoured personnel carrier this fact alone might justify the conversion. Besides, model-making is a hobby, and the conversion can just be done for the hell of it.
The Fujimi kit is also of the B version, and so much of what I say will work if you are starting with that kit. The Japanese kit is, however, twice the price of the Matchbox one, and the wheels are too small for authenticity. On the other hand, it comes with better decals, more detailed tracks, a 37mm gun for the 251/10 variant, a couple of seated figures (although the driver doesn't fit in his seat), and a couple of extra details, such as a towing pintle. Oddly, it has rubber front tyres, too.
Most of what follows is repeated in the text found on the pages detailing how to make the various variants of the 251, but here you get all the tips in one go.
1. Remove the covers to the air vents on the sides of the nose. These are rectangular hatch covers moulded onto the side pieces in the Matchbox model, and separate pieces in the Fujimi kit. Make the area here nice and flat, because later you will need to glue a flat piece here.
2. Remove the bars of the grille at the front of the bonnet ("hood" if you're American).
3. Glue the back, sides, and bonnet of the vehicle together as normal. Leave off the front nose piece.
4. Cut the front parts which would normally receive the angled front nose piece, such that they in profile form a straight sloping line. The slope is such that a bullet hitting the front of the nose would bounce upwards. Try to preserve the little hooks near the bottom.
5. Glue a flat piece of plastic card over the end of the nose, above the hooks, and wait for the glue to set solid. Once solid, trim this piece to be flush with the sides and bonnet, and make the bottom edge straight.
6. Add a small rectangular piece of plastic card to the bottom of the new (roughly hexagonal) nose piece, between the hooks.
7. Drill or burn a hole through the centre of the bottom of the main hexagonal nose piece. This hole was for the starting handle. Alternatively, glue on a small thin circle of plastic card to represent the cover for this hole.
8. Omit the front bumper. The C version didn't have it.
9. The mudguards have several straight sections, such as the long one on which the three storage boxes rest. The front-most of these sections, where the headlamps go, needs to be cut back. Make it about a third of the height it is on the B version, and sculpt a nice edge on it.
10. The mudguard on the C version has a slight (very slight) upward kink/curve in it. Bend the mudguards at the point where the two body halves were joined on the real vehicle. Find this point by looking at the main pieces for the sides of the body. Behind the visors on the side, you can see a vertical line, representing this join.
11. The mudguards have a recess in them, to locate the storage bins. File away the edges of this recess to disguise it.
12. Glue the storage bins on much further back, so that there is only a small part of the long section showing at the back, and so that the front end of the storage bins is near the upward bend in the mudguard.
13. The recesses for the bins may show a bit even after careful modelling. Also, the bend in the mudguard can mean that the piece doesn't fit nice and flush with the side of the vehicle. If so, disguise these flaws with stowage on the mudguard. A roll of cloth is easiest, made with putty, such as Milliput.
14. File the backs of the headlamps into a hemisphere, from the conical shape they have on the B version. I added a slit across the front of the headlamp too. This is not a difference between the B and the C, just a bit of extra detail for either.
15. Glue the headlamps onto the new front edge of your mudguards. In history, the headlamps were supported on right-angled stalks which attached to the nose of the vehicle, but such stalks alone would be very weak on a model of this size.
16. Make big vent covers. These are a strange shape, which pictures are better than words at describing. I used a thick piece of plastic card for each vent, carved with a scalpel to a fine edge, both cutting down from the top side to make the three-dimensional shape, and up from the underside, to make the bottom edge its proper thickness. Study the photographs. Glue these on the upper sloping parts of the sides of the nose.
17. Add a pipe on the left side of the vehicle, which connects the exhaust (which is that odd-shaped part which sits in the lowest part of the mudguard on that side) with the vent on that side. Note that the vent covers overhang the sides, so this pipe can just disappear under the vent cover.
18. With Milliput or some similar putty which sets very hard, fill in the recess on the bonnet where the grille was, and smooth the top surface so that the bonnet appears to be a continuous sheet of metal in that area.
19. Also perhaps with Milliput, make right-angled stalks for the headlamps.
If you do the above, you will have a fair approximation of a C version. Another difference I know about is that the hubcaps on the front wheels were slightly different, but who is going to know? Slap a bit of mud on to hide this if you're worried.
One final tip I will give, has nothing to do with converting the model, but will improve a B or C version: add detail to the edges of the tracks. The Fujimi kit is fine in this respect, but the Matchbox kit has smooth-sided tracks, whereas in reality the tracks had a toothed edge to them. This extra touch makes a very big difference to the look of the complete model, so I highly recommend it. Get a pin, set it in a cork, then heat it in a candle flame, and then use it to burn notches down the edges of the tracks. You can space these notches accurately by putting one next to every hole on the centre of the track.