Normally they relate to build sequence numbers (put there by line workers, in an attempt to keep everything in order) or inspection marks, indicating the item has been checked and approved - or - checked and repair is required.
Most large component assemblies, like tire/wheel assemblies, seats, rear axle trim and driveshaft, front suspension assemblies and engine/trans trim are built up in remote areas of the plant and then conveyed to the main line. It is essential that these items remain in order; if not, then not only is the affected vehicle messed up but everything upstream is messed up too.
They could also be lunch calculations, love notes, or ciphers for their "bookie". Anything is possible.
In the case of the two pictures you've presented - they are not like anything I've seen before on a vintage vehicle. The hood picture seems to indicate perhaps a bank of 10 special parts, and that your's was number 10 in that series (maybe a material tryout of some kind).
And - the cowl photo sure does look like somebody's signature. Never seen that specific one before but I wouldn't put it past someone to do that. In the Pontiac world there was a lady named "Linda" that became quite famous for signing the backside of the glovebox doors that she put together.
Here's mine: I followed my truck down the line (drove it off the line myself) and didn't remember this, but it was apparently build number "87" for that day:
They are no longer there but I would have expected to find a number 87 on the rear diff cover, on the inside of one (or more) tire/wheel assemblies, on the side of the frame and maybe written on the engine somewhere, all in an attempt to keep the flow of material in order.
K