the easiest things are always he speedboard (is it like the Schwimmwagen?) and if there's the later sparewheelholder. The first version has got the long arms which you can see on many fotos from the first cars, this changed in 1942.
The speedboard or controlpanel was changed in the mid of 1943. The most changes had been in the first two years, from 1940 on. But the most cars I have seen are from 1943 and 1944.
Another identification is the hole in front of the windscreen. It's in the middle for the cable of the wipers. The 1944 don't have, the late ones in 1943 have a hole which is closed by a solded sheet of metal. (Maybe also the first 1944ers?, who knows more?) Before mid of 1943 it was used for the elctric wires.
A friend of mine bought an 1945 afterwar car, they are also different. But that's a topic for him, to tell us about.... Greetings
Here's some other external production changes, minus approximated dates and chassis numbers, that I started compiling years ago from the different Ersatztielliste, surviving examples, information provided in some published texts and a couple of fellow enthusiasts. Much neglected though.
1941 Windshield frame widened at bottom. No curves Wiper cable outlet appears in front of windshield Rear fenders lengthened
1942 Spotlight eliminated Dimples appear on left quarter panel; shovel moved to left Turn indicators mounted on towers; horn on side then moved forward Holes appear in rocker panels for new jack Wider rear engine guard New exhaust design plus heaters Plate added in front of spare Wider fuel spout introduced Spokes shortened on spare wheel mount Round taillights no longer standard
1943 Bolts on rear cross member Crossbars added between tow hooks Turn indicators eliminated; mirror now mounted on swivel pin Rear scoop now factory installed ? (This is fairly contentious) Smaller headlights with internal wiring; external wiring soon appears Schwimmwagen dashboard now installed in Kublewagens? (Seems too early) Wiper cable outlet in front of windscreen eliminated Rib shortened on engine/ stowage cover; Rear license plate and Notek light moved to engine cover Windshield frame squared off; some split windshields used Third crossbar added to canvas top
1944 Welded/ Staggered hinges replace riveted hinges Dimples eliminated on right quarter panel Dimples eliminated on left quarter panel Schwimmwagen fuel spout and tow hooks used
1945 Tow hook crossbars eliminated Final units leave factory with single wiper
Here's some of the modifications I haven't been to find any specific reference to.
Door handles changed from flat steel plate to flattened steel tube by 1943. Grab bar behind front seats is welded instead of bolted to side posts by late 1944. Modification dates of three seater. Modification date of rear Notek light. Modification date of step behind front fenders. Wiper motor model changes. Horn model changes. Rear hanging reflector model change. Recess dimples for wipers on front glacis. Mounting holes for turn indicators no longer included on windshield. Mounting recess for front number plate discontinued.