Can anyone point me towards a guide to proof marks on London-proofed gauge or bore rifles? I'm confused by the marks on a steel barreled 20 gauge that's proofed on the barrels and not the flats with London proofs.

Between the viewing proof and the definitive proof, there's a "39" marked on the barrel. No other markings for gauge or caliber.

Any resources you can share or an interpretation?

Thanks,
CJF