That makes sense with the timeline. Holmes Basic and the original hardcover Monster Manual were both released in 1977, the AD&D 1E PH in 1978, and the DMG in 1979. B/X didn't get released until 1981.
Perhaps you were playing with a group which was already playing OD&D using the 1974 rules, and so retained that concept rather than giving Elves class options and making them always split XP evenly between their classes, like in 1975 Greyhawk?
1977 Holmes Basic says
"Elves progress in level as both fighting men and magic-users, but since each game nets them experience in both categories equally, they progress more slowly than other characters." So it requires them to all be F/M-Us like the 1974 rules (doesn't allow for being single- or multi-classed Thieves like Greyhawk does), but makes them always split the XP evenly like Greyhawk.
Zach Howard of Zenopus Archives compiled an exhaustive list for The Acaeum of the differences between the different versions of Holmes, and I don't see any mention of a version which used the 1974 rules for elf advancement rather than splitting the XP evenly.