The latest episode of Roll For Initiative podcast covers Labyrinth Lord's Advanced Edition Characters and compares the LL AEC to AD&D 1e, which is a natural comparison. Now, they mention that weapon restrictions for multiclass characters weren't included in 1e, but they were. The restrictions are buried in the race definitions and actually vary per race:
Half-elven characters who choose the cleric as one of their multi-classes aren't limited by that class' proscriptions upon weapons usable, but they are quite restricted in level. Half-elven characters who choose the thief class as one of their multi-roles are limited to the weaponry and armor of that class when operating as a thief. All earned experience is always divided evenly between the classes of the multi-classed character, even though the character is no longer able to gain levels in one or more of the classes. (See CHARACTER CLASSES, and consult the various classes for more detailed information pertaining to half-elven characters operating within the stated classes.)
When playing a multi-classed character, the half-orc must abide by the restrictions of the least favorable class with regard only to armor.
In the latter case the character (halfling) is limited to the armor and weaponry of a thief whenever any such functions are to be performed during the course of an adventure.
If the character (elf) is multi-clossed, the following restrictions and strictures apply: Although able to operate freely with the benefits of armor, weapons, and magical items available to the classes the character is operating in, any thieving is restricted to the armor and weaponry usable by the thief class.
(maximum of 9th level), a thief, or an assassin (maximum of 9th level). It is also possible for a dwarven character to opt to work simultaneously in the fighter and thief classes; in the latter event the dwarf will be limited to the armor permitted a thief when performing any functions of that class.
It is also possible for a gnome character to be two classes at the some time (a fighter/illusionist, fighter/thief, or an illusionist/thief, for example). In the latter case, the character is restricted to the wearing of leather armor, regardless of which class combination he or she has chosen, unless only fighting is performed by the character.The rules were there, plain as day in the Player's Handbook, but I never realized the small variations the races had in their restrictions. I always thought any weapon could be used at all times by a character if they were multiclassed with a fighter, but it appears I was wrong. (they all have the same armor restrictions if they have a thief class as part of their multiclass)
Half-elves, halflings and elves were restricted to thieving weapons only when operating as thieves - so no backstabs with that battle axe or two handed sword. Or moving silently, hiding in shadows or the like.
Dwarves, gnomes and half-orcs can apparently use any weapon to backstab.
It appears I'm still learning my 1e rules ;)