Well I wouldnāt say I am an expert, but my rules of thumb:
MKI Sprite ā Self explanatory
MKII Sprite / MKI Midget ā Side curtains / no external door handles / no vent window
MKIII Sprite / MKII Midget ā External door handles and removable hood frame
MKIV Sprite / MKIII Midget ā Folding hood / cockpit 4ā deeper
Midget 1500 ā Self explanatory
The problem is that the MK designation does not necessarily define the model. The VIN prefix is really the right way to define them since there was both GAN4 and GAN5 MKIII Midgets for example.
MKI Sprite ā Self explanatory
MKII Sprite / MKI Midget ā HAN6-101 / GAN1-101 ā 998 engine, HAN7-24732 / GAN2-16184 ā 1098 engine
MKIII Sprite / MKII Midget ā HAN8-38829 / GAN3-25788 ā 1098 engine
MKIV Sprite / MKIII Midget ā HAN9-67193 / GAN4-54611 to HAN9-85286 / GAN4-74885 1275 introduced, Midget had vertical grill, both had single rear bumpers and rear marker light below boot lid, center. HAN10-85257 / GAN5-74886 onwards - Facelift cars, Midget lost the vertical grill, flattened tail lights, split rear bumpers, new steering wheel, twin silencer exhaust, rear marker lights inside the bumper ends. To me this is the most difficult to recognize because the facelift was so subtle. It really made the two cars the same except for the badges.
Midget 1500 ā Self explanatory
You posted a while back about a Healy steering wheel in āOriginal Sprite & Midgetā by Terry Horler. This book has all the production changes in it by VIN.