The numbers may be the order numbers..from a sheet of photographs taken at a photographers you were given a sheet and had to order the 'good' photos by the number that was placed on the sheet.
Possibly the first number was the run number used by the photographer to distinguish this session from say 'Herr & Frau Schwarz' who had their photos taken next and were numbered 504.
37 may be the number on the sheet.
Often street photographers stopped people on the street, took their photos and then handed out a card with the day's number on it say 503. You went into the photgrapher's studio and looked at the sheet numbered 503 and ordered from that.
Unless the studio is still in existence you might not be able to find other photographs in the series unless they have been archived somewhere. Here in NZ I managed, once, to find a photgraph from an order number on the back by searching in a museum's holdings where the records/negatives of very early photographs had been lodged