This is the new incorrect alternator it's about 1" too long
As John says, that looks like one from the Lucas alternator series. This page has the Lucas manual which covers the 15, 16 & 17ACR series and quotes outputs of 28/34/36 amps. It notes the 17 ACR model is 6mm longer due to additional windings but doesn't give overall measurements so I'll measure the 17ACR later today and post up.
IIRC the ones fitted to MGBs were higher and more like the 45amp mentions but I don't know what the dimensions are, presumably it's even longer than the 17ACR ?
http://www.lotus-europa.com/manuals/misc/electrical/17acr.pdfBrian
Edit to add : the 17ACR is roughly 15cm long with 8.5cm between mounts and having looked at it again, it looks very much like what you already have. The measurements here were only taken roughly from a ruler held alongside, if it becomes mission-critical I'll take them again with a vernier. But I suspect these are too large for your application ?
Some Japanese alternators are small, the one on our MR2 was tiny in comparison but needed a modern pulley belt rather than the older V belt. You might be able to change pulleys but once you start into that game you might just as well look for OEM ?