each slightly different cosmetics, but brown base, T12 bottle. test good with life remaining

one appears to have '6037' code, darker brown Bakelite

other '537' code, 5881 stamp on dome, lighter tan base

Tubes are tested on a B&K 707 mutual conductance tube tester recently serviced and calibrated by Mike Mabry in Woodland, WA. Mike has been working on this series of testers for many years.

The tube tested as follows, for each side, where the first number is a scaled Gm and the parenthetical number is life rating; a Gm rating of approx. 66 or higher is considered "good": 

537: 93 (90)

6037: 86 (82)

The test ratings on this type of B&K tester may require a bit of explanation. Their "life test" is considered "good" if the meter rating is within 25% of the Gm rating. e.g., a tube with a mutual conductance (scaled) rating of 80 would still be passable if the life test registered a 60 on the meter. Most of their test settings correspond with a 0-120 scaled reading where the "good" area is about 66 and higher on the meter. 

THANK YOU for looking!