Hi,

You could try finding pipe threaded plugs that have a hex or Allen internal socket in the center.
Then you can use an Allen head bit made into socket wrench with a ratchet.

Most industrial hardware places have pipe plugs with square heads.
You can try McMaster Carr or the Fastenal company that cater to plant maintenance departments.

These are used in places where a flush appearance is wanted or linkages or something else has to pass in front of the plug hole.

They are usually made out of better quality steel.
They cost a few cents more but they will a be better choice than any funky plumbing parts down at Billy Bob’s hardware!
The plumbing industry goes out of its way to make things as thin as possible and out of garbage materials. They make things just a little too odd ball so nothing else fits or to be as cheap as they can be for their profit margins. The next thing their out of business or obsolete.

Yep! Square heads are from the of many years past.

""""1980 242 DL with an M45 manual""""

This a a 4 speed tranny without an Over Drive --- The M46 has an OD attached to the rear.
.The M47 is a true 5 Speed found in 1987 and later 240s


Both can use Redline MTL or the Volvo Manual Trans Fluid ---which are more or less the same specs.

If your 1980 has an M46-- with a OD, then what Dave says AT trans fluid - is what you should use... because of the OD. And this would be different than the fluid for the M47.

I have a 1980 DL with the 4Speed, and an 87 245 with a 5sp

http://www.vivaperformance.com/genuine-volvo-manual-transmission-fluid/

https://www.ipdusa.com/products/6980/106979-redline-mtl-manual-transmission-oil

I drive a 1980 245 M46 also. I can't tell you the thread (I think it's like a pipe thread) but what you need is the proper tool. You can purchase an 8 point socket and have no need to change anything. A 7/16th" size socket should work just fine. If there's such a thing as an 11mm 8 point socket that would work too. OR -- back in the old days when square shaped drives were common (tranny and diff plugs, brake shoe adjusters, etc) you could buy a wrench with boxed square ends.
I've always used a normal Type F auto trans fluid in mine (310k) but I would think a compatible synthetic would be ok -- unless -- the synthetic is not compatible with the OD conical clutch. Maybe someone with experience with that aspect will post. Also -- I think you meant to type "M45" - not 47 in your last sentence. -- Dave