I'll check the outer tubing dimensions this evening. The bike is a roughly 1972 Jupiter Beat Super Deluxe, which is not on that list. Every reference I can find to that brand and model mentions hexagonal tubing, but this has all round tubing. Paint and decals, except for the checkerboard stripes, are identical to this one. As far as I can tell, I think all the original components match that one as well.

Edit: I don't have a better caliper handy. At the bottom of the lug, the R-L measurement is 1.114", about 3" below that at the top of the decal it measures 1.128".

On closer inspection, that slot does look a bit uneven. I bet it is supposed to have a bigger seatpost.