[MENTION=425004]merziac[/MENTION] - That port is threaded.
Thanks for the encouragement!

I was thinking of a fine screen with floss to pull it into place and put the epoxy on it and then cut the floss off when dry. Of course, the edges of the hole should be clean for adhesion. Like patching dry wall.

This is somewhat of a mystery frame set. Bianchi didn't sponsor any TDF teams in 1971 that I could find. Here are some frame and fork features in pictures.
Note the "L" stamped into the crown. It is also on the BB shell. Indication they go together.
P1060046 on Flickr

P1060038 on Flickr

Shell goes right to the limit of the CS
P1060039 on Flickr

Long tangs on the fork
P1060036 on Flickr

no spiral in the steerer
P1060045 on Flickr

Campagnolo DO not Bianchi
P1060037 on Flickr

P1060041 on Flickr

P1060040 on Flickr

Can't tell what is going on with the dimple. Like it was pressed from the inside toward the BB shell.
P1060044 on Flickr

The other side of the seat cluster
P1050693 on Flickr

The ST takes a 27.2 post and the frame is completely chromed.