Thank for the feedback. It was purchased along with this bolt below from Apex last week. It was apparently there last L2A3 bolts? Here are the markings on the bolt."F55" stands for Fazakerley 1955. ROF Fazakerley produced Stens and Sterlings in large quantity and it's not uncommon to to find the manufacturer and date code on various parts. It looks to be a pretty early specimen.
I can't make out the other markings, but it appears to be an inspector's stamp.
Apparently this was the last Sterling bolt they had for sale. All the others had that normal 10 digit or so number on it. I guess they saved the best or worse for last?That's interesting. I recently bought an Andrewski/DLO Sterling and picked up some parts including a spare bolt and several spare extractors from APEX. Neither bolt has markings like yours and none of the extractors are marked or have that channel. Presumably this is just the difference between Fazakerley and Dagenham guns/parts? I have the Guns of Dagenham reprint but haven't started on it yet, I'm still making my way through Dolf Goldsmith's Browning books.