Interesting, that Grim Reaper thing. TRO3055 doesn't mention the Kintaro in the text but between the weight, similar armor distribution, the SRM and looks, the Grim Reaper could very well be based on the Kintaro blueprints.
I would assume the Kintaro wasn't in production in '55, but presumably its old assembly line still existed on Terra, probably mothballed. Restart the line after modifying it a bit to produce Grim Reapers instead of Kintaros. Considering ComGuards needed new 'Mechs ASAP, modifying an existing design would be faster than a brand new design in a brand new assembly line (or making extensive retooling for another line). As for why a new 'Mech, parts availability is reason enough.
Certainly something i'd regard as headcanon, it makes too much sense.
EDIT Of course, there's a catch. I was going by info on TRO2750 for the Kintaro. TRO3050U places the primary factory of the Kintaro-20 to Mars, implying it is being made still. That said, minor and secondary factories aren't covered in TROs, so there could have been an unused Kintaro factory on Earth that was modified. Or alternatively an existing factory was modified... presumably assembly lines are semi-modular or reprogrammable even if the process isn't quick. Anyway, the Grim Reaper could still be based on the Kintaro, factory-issues aside.