One of the more intractable problems for an AD&D e1 DM is the addition of a multiclassed character. They cause a variety of problems. First and foremost, it creates a capability challenge for the DM to work against. Having to plan for spells and abilities is hard enough, but when you have a Magic User and a multiclassed F/MU you might forget the party can do something twice.
This problem could be its own post, so I'll rein it and move on to the commercial. I mean the character. Or both.
I am using my own character sheet which you can download from DriveThruRPG for Regulus, the Fighter/Magic-User/Theif. This Regulus is one variation of the same basic character I used for a friend's campaign. I was the only person who didn't know we were in Narnia and died a zillion times. It got so bad that I stopped making up new names.
Not bad for a sheet designed by a committee.
Moving over to the next part, we have class and level information which also has enough space for multiclass information:
Beyond this one time, it was a mostly ignored stat. It's too complicated and extraordinarily dangerous for Illusions and Bards to have.
(Unless a sneaky person has 2 or 3 bags of holding...)