GE = the Philips code for EH90
1 = change code. Over time, as Philips implemented various changes and modifications to their tubes, the change code was incremented. I would guess that most of the time, these changes were small and perhaps undetectable to the naked eye.
A good example is the Philips E91H/6687 manufactured in Heerlen, Holland, as I am somewhat familiar with four different pairs:
3L = E91H
Date 1957 = 3L3 (D-getter and pinched glass envelope)
Date 1960 = 3L6 (D-getter and straight glass and ????)
Date 1962 = 3L7 (O-getter and straight glass and ????)
Date 1968 = 3L8 (?????)
Another little detail to confuse matters even more : my first pair of 'Siemens' EH90s (without 'made in germany' on) had code B6G3, which is 1966, ya? The second (WITH) have B5I4, which is therefore 1965...Now the first have plain GE above, the second GE1 - one would have expected the change code to be the other way round , non? My conspiracy theory moves forward another step!!! (There's probably a very simple explanation...???).
Audiofanboy - glad I wasn't having a 'bad ear day' re the bass on these babies...when I returned once again to Cat Stevens's remastered 'Hard headed Woman', it nearly blew my head off!! So yet another added bonus - even if they aren't actually Siemens/Germany made...Will be interested to see what you make of the Blackburns after further burn-in.