Does it correlate in any way to the serial number on the barrel or breech?
FWIW, I have two 618, one 620, one 622 and one 624, from the early '60s to early '70s, and they all have the ser. # on the breech block. I think the stamped-on-wood numbers are from a more recent period, but don't take my word on that.
I have two 618's that have the number stamped on the stock.