Reversing polarity was a simple way back then to make a phone be used for incoming only. If I remember, the later dials were labeled OPG for Opposite Polarity Guard and those dials eliminated the condition.
Besides, If the phone doesn't work where you send it to, just tell them their wiring is backwards. They will go nuts when you say that but you will be right. You will then see what kind of customer response some us have to put up with!
FYI - Here's a freebie. Since you like old phones, you may eventually encounter an install with rotary phones. If one is wired backwards, it can also jingle when another rotary phone is dialing.
