KT 91

I believe like the Gold Aero KT99 it is a KT90. I also assume they are made by Ei. If so the answer is yes. I used KT90/99 in place of 6550 and KT88's.
Do they look like any of these?
https://web.archive.org/web/20110426202420/http://www.manleylabs.com/galleria/KT90.html

These are KT 91, probably the same as KT 90.

The KT91 must really be an oddball tube number. Doing a search I couldn’t find any data sheet on it. I did find an Ebay listing for the tube. The guy says he used the 6550 settings on his B&K707 to test the tube.

http://www.ebay.com/itm/Vintage-Audio-Research-KT91-Pair-of-Tubes-Tested-95-B-K-707-RARE-6550-Versio...

I see the pair were made in Yugoslavia.

Yep, they're an odd-ball for sure.

Is there anyone out there old enough to remember going to the corner drug store to test tubes? I had such stores on all four corners, just two blocks from my house when I was a kid. Always loved tubes, still do.