Ref bolt roots, there are patterns. For byf, in general (yes, there are exceptions):
pre-1942: polished with few, if any, tooling marks. serifed font.
1942-1943: filed with obvious linear tooling marks. serifed font. lots of MI (FN-subcontracted bolts) start showing up.
1944 to the end: either swirl marks from milling or linear tool marks, depending on serial block. Sometimes unseriffed serials are used, particularly on FN bolts the later in the war we are talking.
You can't always rely in the trough to determine originality. On reproductions, I've personally welded up the rood and re-machined it to original specs, then stamped a new number. I also have a set of lowercase MO-like stamps. I don't make any repros of basic infantry rifles, but others do. (I stick to sniper variants and leave flaws on purpose to evidence the fakery).
When a bolt stamping is shallow from the factory (often), I will simply put the bolt in the mill and use a carbide end mill to mill off the old marks, at a very slight angle so that the trough stays original depth. Hard to detect in a photo.
Here is the 44byf zf41 bolt root I faked in another cgn thread. If you relied on the trough or the belief that lowercase late-war suffix fonts aren't out there, you COULD be fooled. In this case, original numbers were removed from an RC bolt using the milling machine.
And here is a very late (byf44 k block - november 44) bolt where they went back to serifed font and more linear tooling marks, very like the 43 shown above.
Here is a byf41 where you can see the original linear tooling marks have been polished down significantly. seriffed font.
