Interesting.
Just my thoughts but it might be a old landline phone number.
I remember in SW Missouri many decades ago, actually before I was born lol, phone numbers would be for example "UN1-7820" which would be 861-7820. The UN was short for University.
That person would say "call me sometime at University 1-7820"
Or M54-5555 which was McDaniel54-5555 would be the phone number of 654-5555.
The letters was the CO (Central Office) or I other words the service area.
Your "J" might be the owners phone number, 586-3186.