Hi,
Guess there's no system (we know of) indeed ! My 2 cents.
'94 TiBolt 17,5" : RM17xxx
'96 Blizzard 17,5" : B 175 T 9507 xxxx
'97 Element TO 16,5" : E17xxx
TiBolt : (R)ocky (M)ountain, (17),5". Since the frame was welded by Sandvik ,in that period they supplied other brands with Ti frames too, it makes sence to tag prefab bottom brackets with information about brandspecific tubesets/geometry. xxx could be serial#.
Blizzard : (B)lizzard , (17),5". (T)ange tubing welded in july 95 ? xxxx could be serial#.
Element : (E)lement , (17) could mean 16,5" since the measurement on FS frames differed from HT ? xxx could be serial#
Cheers,
Ben