I see two adjustment screws on my 1975 Motobecane derailler.

It looks like the one on the end is the only one of the three I should adjust to tighten the chain. I assume clockwise tightens it which I was trying to do to take the slack out.

Tighten the chain...

the B screw, closest to the axle will impart some "tightening" of the chain.

the majority of the chain tension will be in the jockey cage spring- those can wear out and essentially are a cause for derailleur replacement, springs possible to exchange, doubtful to find.

The B screw (the one farthest to the left in your photo) is not used to adjust chain tension. It’s to adjust the distance between the jockey wheel and the cogs. If your derailleur isn’t tensioning the chain properly, it’s probably time for a new one (unless you can get a spring and feel comfortable replacing it).

It seems ok now. No performance issues except that the chain goes off gear too easily when shifting to least tension.

Also I have been using chainsaw oil for lubrication, which is a big improvement over the WD40 which is more or less a solvent.