I am half way through researching this issue---I am currently on fire contract with the USFS, (and carded for mountain flying), and sent this problem up the chain of command to Region 4 head office. The response so far is that the definition was changed in the last two weeks. It now states in FSM 5710.5 that mountain time is defined in 14 CFR 95 subpart B.
I will add the folks here are not happy---basically, as Wulfman76 points out, the map in subpart B contains areas at sea level. This "may" be changed back to what it was---which no-one can find but it was above 5,000 ft we believe.
Watch this space for updates.
14 CFR 95 Subpart B