Colt 601 Build

[Madcap]

The buttstock arrived this morning and looks very good. The sling swivel was removed and the stock lightly sanded with 180 and 220 W/D.

The tab that inserts into the receiver has a hole with some sort of material that I assume was stuck in there for some reason and I'm confused as to what it is or the purpose.
Photos attached are the best description...anyone also know what the purpose was for the hole??

edit:
Help...! The realization has struck me that I'm not sure where the detent spring for the takedown is supposed to go on this stock....is this the wrong stock for a 601?? Something doesn't seem to make sense.

Dick