Though I've never had to use that type of recovery before, it is my understanding that you can do a restore to Custom firmware by putting the CFW install pbp and the associated files in that folder.
To restore to 3.90 M33, I think you just have to download the original 3.90 M33 release, extract it's eboot.pbp to /GAME/RECOVERY/eboot.pbp, and place the necessary 390.pbp file in the same folder.
(390.pbp file being the original firmware update eboot renamed, available here if you don't have it:
PSP Firmwares )
Anyone want to confirm or deny this?