Anyone identify what this Speedmaster case middle was ?

http://www.ebay.com/itm/231471570735Purchases made through these links may earn this site a commission from the eBay Partner Network

It was fitted with a 145.012 caseback, but it was obviously not that model.

Was it an earlier straight-lug speedmaster, or a different 321 chronograph model ?

Looks like it was in nice vintage condition, whatever it was.