~ 4" cast iron pipe rises to 1st and 2nd floor bathrooms

Pipe drops to basement floor into a cast-iron double-hub [looks like a figure 8] with a cleanout cast and threaded right in the hub casting next to the hub that holds the stack. It is not a threaded nipple caulked and leaded into the hub.

Problem is the cleanout plug that we need is an oddball size. It is in an 80 yo house in Detroit. If I were to guess, it would be an 3 1/4" IPS - the ID of the threaded hole is 3 5/8"

The P56-400 [4"]Fits-ALL tapered Lead thread plug is too big. The P56-350 [3.5"] is too small as the lower OD is 3 3/16" and upper OD is 3 1/2". For some reason Jones Stephens uses actual inch measurements, not IPS sizes for these items. What would do wonders would be a taper from 3 1/2" to 3 3/4".

A 3" brass cleanout plug drop through the hole. A 3 1/2" brass plug is about 1/4" too wide to get a start on the threads.. hence my guess that it is about 3 1/4" OPS but no plumbing shop I have visited ever heard of that size.

I suppose one of those plastic 'test' plugs would work to seal the cleanout hole, but I am not sure the inspector would approve that for a permanent fix.

I'd hate to have to tear the concrete floor out to replace the double hub.

So where does that leave me?

May be you could take the 3 1/2" plug to a machine shop and have it turned to the correct size.