You are correct and there are only 31 days in January.

Not going to call it a proven fact but it is looking like

Prefix on the 69- later examples is a shift or whatever designation.

The next stuff is the date.

Suffix "A" is for the manufacturer "Arvin".

This is a work in progress.

The pair in post 7 are small block also.

Direct in hand comparison between one of hers and one of the NOS ones.
Same other than weep holes and nipple in the inlet end.

FWIW we have a repop exhaust for one of the converts
The pipes fit the repop muffler.
They do not fit this.

Sorry about the other mufflers the seller had.
Looks like this pair were the only BB ones.

Enjoy or ignore- your call but I am satisfied with this info.

Pics 1 and 2 show markings are same font
Pic 3 shows weep holes in replacement top and bottom Makes it a R or L.
Pic 4 shows same construction.
Pic 5 shows double wall head pipe.
Pic 6 and 7 show same construction with the service part having a nipple spot welded in where the head pipe would be welded in on a factory setup. Same muffler other than that.
Pic 8 shows comparison of marking placement.
Pic 9 shows the rippled seam is the same.