Do not use an older version (like 0.168) with a 0.251 ROM set. The cryptography and memory maps have changed. Download the official MAME 0.251 binary from the official MAME developers' site or trusted compiles like MAMEUI64 (version 0.251 specifically).
Like all MAME builds, version 0.251 is —it's a reference framework. The "Full ROM Set" refers to the complete collection of ROM dumps, disk images, BIOS files, and device software that MAME 0.251 expects to run every possible game and system it supports. Mame 251 Full Rom Set