Eyepiece / Eyecup for FSU Turret Viewfinder?

[kovacs22]

I see that there's maybe a 2mm threaded area for an eyepiece or eyecup for my FSU universal turret-style viewfinder. I'm assuming that's for an eyepiece / eyecup. Is there a rubber one that's made to go with this particular viewfinder? Or, does anyone know what size or other aftermarket versions will fit? If there's an eyepiece that fits and was made for this FSU viewfinder's contemporaries (i.e. same era), then that would be ideal. But most any eyepiece that fits will be good to know.

Thanks in advance!