Are you sure there wasn’t a word ahead of the numbers, and you used the first 2 letters to transfer to the phone number? When we first moved to CT in 1956, the first phone number was DIamond nnnnn. That gave you 3,4, then your 5 numbers. I would give the number but it’s still in use at the old homestead, now owned by siblings. In fact, I retired that old rotary phone and it’s still working in my late fathers room. My sister still has my father’s original license plate too. We don’t throw stuff out just because it’s old.