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.