If the file name is in either the PPS or the rgsproc formats, then rgssources is able to deduce the instrument name from the file name. The PPS format is PooooooooooiiteeeSRCLI_0000.FIT; the rgsproc format is rrrrooooooooooiiteee00_sources.ds. In these templates, rrrr is the rev number, oooooooooo is the observation identifier, ii the instrument (either R1 or R2), eee the exposure number and t the exposure type (either S or U)). However forcing rgssources to do this is not the preferred option, which is rather to give the instrument name directly via the parameter instexpid.