From Alex Knight's page on the Classic series LEDs:
"In mid-1997, Arrow Electronics, a major components distributor, still had a supply of the HP# 5082-7405 LED modules, at a price of approx. $25.00 U.S. each.
"That's the good news. The bad news is that there is a significant variation on the brightness of different lots of the LED modules, and each module is marked on the bottom with a letter indicating the relative brightness.
"Some modules I obtained that had a date code of late 1996 were marked with the letter "W" and were significantly brighter than the modules I had pulled from a 1973-vintage machine that had modules marked "J", which were themselves slightly brighter than modules marked "H" from a 1975- vintage machine.
"Therefore, if you decide to have a module replaced, you will likely either have to live with having a group of 5 digits at a different brightness, or to buy three brand new modules at a cost of about $75.00."
(AKKnight, w/Joe Rigdon)
http://aknight.home.mindspring.com/clasdisp.htm