[old_tv_nut]

It's hard to guess how this would be used for everyday repair/adjustment work. If it were a service instrument, it seems to me it would be marked as such, not with the Lancaster label.

It appears the red and blue pieces are rotatable and the green is fixed.

My wild guess: an instrument to measure center convergence of the tube without any static or dynamic convergence correction components installed, to see if it is within specifications. It would be used by TV manufacturers who bought tubes from RCA.