B31ZB Engine number puzzle!

[Minns]

Puzzled by the engine number on a BSA 500?  I was offered yesterday.   The stamping of the 'GS' looks weird and I've never seen a factory stamp font like that.  I'm wondering if the vendor is trying to fake the 'Gold Star' numbering?   Any help would be gratefully received.  Thanks

[JulianS]

Clearly a ZB31 stamping which has been subject to rather naive tampering with by adding the G and S either side of the original full stop.

[Derek996]

There was never a ZB31GS. If it was a Gold Star it would be marked ZB32 GS xxxx. There was also an iron engined B32 (I have a ZB32) and and alloy B32A, which gets confused with the Gold Star.