The numbers you're referring to are almost certainly printed on the back of the paper by the photofinisher, not by the photofinisher. They may be codes relating to the color balance settings used in the photofinishing machine, codes to identify the facility where the print was made, etc. Perhaps somebody here with experience in the industry would know more. I know that when I get digital prints made at my local minilab (at a Walgreens store), I see two lines printed by the minilab. The first includes a number whose purpose I don't know, the filename of the file I submitted, and a print number (such as "22/34" for print 22 of a 34-print order). The second line reads "Walgreens 03898" (presumably the store's ID number) and the date of the order. I don't see anything of the form you're reporting, so it's likely something different.
Manufacturers sometimes put backprinting on their papers, but it's usually restricted to the company name, sometimes the product name, and occasionally a "do not copy" notice on "professional" papers (ironically this is the sort that most home darkroom users can buy).
