This is normal. Since you updated to 3.73, the syscon was updated with new hashes aswell preventing 3.56 from running.
Flash 3.73, boot console and use the MinVerChk PUP. If it says minimum version is 3.56, get JFW DH.
PS3 Slim CECH-3004A 160GB (500GB)
PS2 Slim SCPH-70004 - FMCB 1.8b