That's a pretty good guess. I saw a touch-tone wall phone that's still hard-wired (non modular) where Ma Bell put it and still has its original number card (this one happens to be a card, not a sticker), and sure enough there's no M on it. This phone is in Northwestern Bell territory and was probably installed between 1967 and 1969 based on it being touch-tone and not having the 1969-present Bell logo on it.
If it stands for Modular, I wonder if it's sort of a disclaimer, since the reality of a modular phone is that it's far easier to move it, therefore there's more of a chance that the number card/sticker could be wrong if the customer connected the phone to a different line.
I d> Allen Newman wrote:
> On the number cards/labels affixed to latter-decades' Bell System
>> phones, there was a letter M stamped like this:
> Could it have meant "modular" since that number card was intended for
> modular phones installed by the customer? They used to give them out
> at Phone Center stores.
> I didn't care for them since it was a sticker, not a card. Admittedly,
> for most people that what was best. However, since I knew how to open
> a dial, I wanted a card to mount behind it and didn't want some sticker
> fouling my dial. Also, they used a stamper that was in relatively
> small type size compared to the bigger size used by traditional
> installers. Of course all they had was all-number, no letters. In our
> area, we were still using letters in a limited manner. To this day,
> the official internal identifier for telephone districts in our area
> was the old exchange name from way back.