But congrats on your 857 first of all!
I have no information as to whether the 857 is a legitimate piece
to feature a movement that is signed different from the dial. But I
can hardly imagine a reason why a Lecoultre signed piece would
originally have a movement that is signed JLC. There have been
JLC watches where the movement was signed „just“ LeCoultre for
a certain reason, but the other way around looks strange to me.
A good first step could be to find out whether the movement and
case are from the same period of time. If you send in the case
and movement number to Zaf Basha’s vintage forum you will almost
certainly get an answer on that question. Presumably Clavi could
answer it too. Maybe it is just the dial that has been replaced. Maybe
the watch is all original. Yours has an uncommon brezel too. Does the
brezel turn? Marking it with minutes as on the dial instead of hours
would not make much sense if it did not. The brezel on the regular
857 is not supposed to turn, as you will know.
Cheers
Christian