In internal memory naming it gmapprom.img, and the other 2 in SD under folder Garmin naming them gmapprom1.img and gmapsupp.img.
Here are the mapset files supported by Garmin Nuvi 255w GPS navigator:
gmapbmap.img - the default base map
gmapprom.img - the primary mapset
gmapprom1.img - the secondary mapset
gmapoem.img - the OEM mapset that cannot be unselect by user (if it exist) lo
gmapsupp.img - the supplemental mapset which you can have one in the GPS flash memory and another one in the attached SD card.
The nuvi 1xxx series has the advantage/ability to load any number of maps from it's internal memory. Simply create a folder in the root of the nuvi drive called "Map" (no quotes) and copy your desired map into that folder. The map does NOT have to be called gmapsupp.img nor any of the older nuvi naming conventions. In this case you should give it a more descriptive name such as Topomaps.img (or anything else for that matter). The only stipulation is that it ends with the .img extension.
The trick works on SD card too.