Magic-users may use any kind of non-magical weapon or armor.
Credits for this really have to go to "the Norse" (real name sadly unknown but I would love to add it to the credits for TFG) who sighted the final piece of the puzzle - a semi-colon in the class description for magic-users that easily is overread.
The rules clearly state "The whole plethora of enchanted items lies at the magic-users beck and call, save the arms and armor of the fighters (see, however, Elves); Magic-Users may arm themselves with daggers only".
The first part of that definition before the semi-colon clearly restricts magic-users from using the enchanted arms and armors of fighters ("The whole plethora of enchanted items lies at the magic-users beck and call, save the arms and armor of the fighters", underlinings mine) - but nothing more. Non-enchanted items are not mentioned in any way and thus any such interpretation regarding not being allowed to use non-magical arms and armor (up to this state of the rules) only can result from using preconceptions from later editions of the game. Which do not exist in the spring of 1975 and thus are out of the discussion.
The semi-colon is the missing link in the next part of the sentence - as explained by the norse it clearly suggests a close grammatical connection to the previous statement - thus meaning that magic-users only can use magical daggers if they are going for magical arms and armor, nothing else.
So I rest the case for now. Magic-users in TFG will be able to use all kinds of non-magical arms and armor although I will add a sentence to the rules implying that they in most cases will be far less comfortable doing so than fighting-men - and the referee can make of that whatever he wants.