Charlie,
I recall late last year an RCA CTC-20 Roundie was offered on e-bay. Item: 2197267225. (This can still be brought up). It was touted as RCA's last roundie produced in 1968. The seller speculated that RCA was using up its stock of 21" round tubes. Maybe this is true of your Philco as well. The RCA CTC-20 on e-bay was based on the 25" CTC-38 chassis. Does this make it a hybrid set?
Steve