XHRM owned by the Rivas-Kaloyan family was one of the early FM's out of TJ....they even are granfathered at 4 call letters as compared to most Mexican FM's which have 5.
Lots of Mexican FMs have 4 letters, and some have 3. The reason that there are so many 5 letter calls is that the first two letters are defaults, XH. So to do anything meaningful, concesionaires asked for 5 letter calls like XELTN for "Latina." But even in Tijuana, we have XHFG, XHA, XHTY, etc. But since stations seldom promote in Mexico using the calls, it's not too meaningful a distincytion.