Jan-Martin Noeding, LA8AK wrote:
Surprised to obtain the A****er Kent 137P where some of the tubes have
blue glass, type number 56 and 58, but not every 58 has blue glass.
See http://home.online.no/~la8ak/q2.htm

73
Jan-Martin
LA8AK

Arcturus made a lot of tubes with blue glass. There were some other
smaller makers also. No real significance (marketing gimmick?) but
collectors like them because they are somewhat interesting.

-Bill