The Model 51 was first introduced in 1968-69 and the last was built in 1990. The last one (E-52 Summit station Sequoia National Forest) was retired in 2005. There are still a few being used by the USFS as reserve engines.
Sierra National Forest Model 51 Ford F600.
Credit Sierra National Forest archives
Sierra National Forest Model 51 Ford F600.
Credit Sierra National Forest archives
Aaron Woods