@E.: I was to lazy to look it up. Yes, as far as I recall the letter in those days indicated the number of elements. So this is an "A" or even "Pre-A"? Serial number 400253? I think Nico van Dijk and Richard de Stoutz have some lists of serial numbers and production dates or years for bodies and lenses. Very helpful, inasfar as it helped me find out I've actually got two Ai's besides five Ai-S's, a few of which are relatively young. Some Ai-S's are still made.

Interesting stuff!