I think the M871 was the replacement for the M127. Higher payload and ISO container locks. Here is what the manual says for the M871. (Kingpin is 2")
Do not tow the semitrailer with the M52, M52A1, or M52A2 truck tractor. The
M52 five ton truck tractor’s inherent design capabilities are not compatible
with the semitrailer, and if used would result in a serious compromise to
the safety of personnel and equipment.
When towing the semitrailer with M818, M931 series, or M932 series the fifth
wheel wedges must be in the locked-in (pushed in) mode for highway and
secondary road use, and in the locked-out (pulled out) mode for cross
country operation.