Ok, well after some measuring, 25 series are all the same, 10 or 11hp. Now 12hp 28 series is larger at the rear end, fits you're measurements, but is 10mm longer overall. It achieves this with the mounting points on the head being longer.
So what i am suspecting, which would fit with the cover being missing and having been rebuilt, is that someone has put together a bitsa, with a 25 series head on a 28 series block, probably for a good bore, then gone to fit the cowl, with it not fitting given up and sold it off.
I would be pulling the head and confirming the condition of the block, then if good then looking to source a complete wrecked 12hp and making it all correct.
