Well for a start there is single and twin armed original wiper motors. There is the angle of the dangle cleaned each stroke. There is the Italian one, the French one, could be an Argentine clone still made as there is for the Regulator etc. and I have no doubt that BMW/Bosch changed spec at some point in construction, probably earlier, rather than later. Then there is aftermarket units of very similar style from aftermarket Bosch rear wash wipe kits. These were then copied in the East and flogged with various names on but are effectively clone copies. Lastly you can fit later wash wipe units that have taken the Bosch spec and fullfil the roll cheaply without being correct at all. As far as I am aware none of those units will fit a Messerschmitt without modification. The key one being reaming out the hole in the canopy. Then if you want to put a Schmitt motor back on the thing is as loose as to be useless unless clamped up tighter than it should be. That then eventually leads to the motor base failing or undue tension on the wiper spindle which takes out the weak drive gearing. Do not do it. Bosch motors are oval, Schmitt ones are square shaped, they are not the same.
Schmitt motors are faster too.