I went and had a look last night, and though I know very little about 1275's (all of my Mini's have been small bores) I'll post what I noted.
EDIT - Added items in Blue
As noted has the extra Stud and room between Rocker cover and Thermo Housing.
The Welsh Plugs are Cup Type, not Convex.
It has Side/Tappet Covers.
The Engine No was as listed.
It is a 1962 850
It is currently registered in NSW (till September I think) and has been for about 5 years going by th AA ** ** Rego.
It has had a Nissan/Hitachi Distributor Fitted
It goes like the clappers
It has a fairly hefty Cam, but if you keep it up on the power band, woo-hoo.
The Gearbox was the nicest Mini Box I have ever used, short throw, precise, very nice.
Yes, I took it for a nice "test drive" at the encouragement of the owner.
It has Minifin Alloy Drums on Rear.
Cooper S Disk Brakes at Front.
Adjustable Lower Arms.
Doesn't have Hi-Lo's
The Drivers seat is a bit loose, will need to check the mounts
It appears to be pretty well rust free, small ding in Drivers Door, there is a small split in the passenger floor, but seems solid around it.
There is paint cracking off the boot around/under the No Plate Flap
Might have forgotten some bits and pieces, but all in all seems like a nice car and worth a look. As stated it is registered in NSW so would have had to go through an Identity Check process, so not sure what the implications of the Engine No would be in other states. In NSW it has already been accepted though.