The marks on a US manufactured bolt head are proprietary, or assigned by a body like SAE or ANSI. In 1969 there were 490 bolt manufacturers in the US. Several had more than one plant and assigned a separate mark to each.

As far as reproductions go, I do not believe a reputable US bolt manufacturer will produce a bolt with a marking not his own. Thus, it may be nearly impossible to obtain a new, currently manufactured bolt with the correct mark on the bolt head.

Not what you wanted to hear, right? Maybe someone else has a source.

R.