That is a seriously old ballast! No doubt a re-branded 80w mercury ballast! That 4 pin starter and holder really completes it along with the old rayon flex! Are any of your 4 pin starters the old thermal type?

It's got to be 1940 or 50s, one of the very earliest. It's rather quiet too and doesn't get too hot, I should think the size of it has helped it last so long, the big heavy case should take the heat away from the windings. Unfortunately all 3 of the starters for it are the later glow type, the glow bottles inside are about the same size as a 2-pin starters though!

Do you by any chance have a top down photo of the ballast's label?

I've just uploaded one

LOL - One of Eliot's early uploads!