I have listed them as new because of appearance and measured Gm as new tubes. The Gm, as measured on the Weston 981, are both 6150 micromhos.

They have both been tested on my Weston 981 (type-3) tube tester which tests for Proportional Mutual Conductance. 

The 5881 tubes are similar to 6L6GT tubes, but not exactly the same.  They have the same footprint and the specs are similar. 

The 5881 is a more rugged tube made for use in Military equipment. The 6L6 is more common for audio amplifier and guitar amplifiers. They do not sound the same. Some say the 5881 sounds better to owners of guitar amplifiers. Audiophiles may say the 5881 has a better bottom end and more transparent than a conventional 6L6. Since the specs of a 6L6 in not identical to a 5881 tube, it is recommended that you re-bias your output tubes if you replace a set of 6L6's with 5881's. The Gm of each tube will be written on a sticker on each tube. 

This sale is for two tubes. Check my 100% feedback since 1998. Free shipping.