This is matched quartet of Raytheon RK 75 (307A, VT225). This
is very rare version of 307A, with just three ribs, all other versions have 5
ribs. This tube is also characteristic for double symmetrical getters.
Click to see All 307A VT-225 306A in stock
how much were these worth? I have 5 matching tubes in original packaging. 3 ribs. dated Aug 31 1944