Allow me to clarify...
I posted the details of the schedule that the A300 has during the peak week of flying. However, this is not necesarily what the aircraft will fly every week. Indeed, there may be weeks when the B757 is used (such as at the begining and end of season). I can see at the moment that on the 1st and 8th May the MON1786 is scheduled to be on the B757 and is indeed to PVK.
There is a separte flight to CFU on the 1st May which is also on the B757 and is MON3690 LGW/CFU. However, this is different flight.