C15 piston decode

[Tigerfeet]

Having got started on stripping the engine, I was intrigued by the markings on the piston.

It appears to have had some enthusiastic decoking during its life - the cylinder head is pristine.

Does anyone decode the stamped markings.  It seems to be:
40/633 BSA
       m/m OS
8/6/72

I'm intrigued that it appears to have an oversized piston measured in mm, but preceded by a fraction?   

[JulianS]

40 633 is the part number for the bare piston (40 632 in parts book is piston with rings)

It is 8:1 compression.

8/6/72 is the date of manufacture.

See below for the part number. You can see the compression ratio in the parts book.

[Tigerfeet]

Thanks Julian.

And I see that '+1/2mm'  is effectively '+20' thou, which is the units I would have expected.