You may find this site from Brent Jesse Tubes helpful. I found this on there.
B749: This is made by Genalex (Marconi-Osram Valve Co. of England) and is extremely rare in the USA. The B749 is in the “Gold Lion” series of audiophile tubes, and has the gold lion and gold script on the glass. Inside it looks similar to a Mullard 12AU7, and has a red Genalex decal across the bottom. These are said to sound the best of all 12AU7 types, and this reputation, coupled with it’s overall scarcity, has driven the market price of this prize tube to extreme heights. Worth seeking out as these may be the first out-of-production classic European tube to go totally extinct over the next dozen years or so.