The 737-200C was the designation for the all cargo model. Boeing offered a 737-200QC ("Quick Change") that could be either all-cargo or all-passenger (the seats were on pallets so they could be rolled on and off as needed). With the 737 Next Generation Family, the "QC" designation was swapped for "C" - the 737-700C Quick Change can be operated either in an all-cargo or all-passenger configuration.
I do not believe Boeing ever offered a factory-built 737 combi model (that could seat freight in front and passengers in back) nor did they offer a freighter model (sans 737-700C). As such, all 737 Classic and Next Generation Combi and Freighter conversions were done aftermarket. So all the 737 Classic and Next Generation "C" combos are aftermarket designations by the conversion companies (737-400C and 737-800C, for example).
As such, I would guess the Canadian 737-200Cs are aftermarket combi conversions of the original 737-200C all-cargo models.
One note, Boeing is considering offering a combi version of the BBJ
(based on the 737-700C).
As of 1978, the 747-200B Combi was the designation for the model that had passengers in the front and cargo in the back. The 747-300 and 747-400 versions of this were known as the 747-300M and 747-400M and I believe Boeing retroactively renamed the 747-200B Combi to 747-200M as that is how they now appear on the Boeing historical Orders and Deliveries website.
The 747-200C Convertible was the model that had the reverse - cargo was loaded in the front (via the nose door) and passengers were loaded in the back. It could also be flown in an all-passenger or all-freight configuration as needed.
As for Nepal's 757-200M, was it originally delivered in a mixed freighter plus passenger configuration with a partition between them that was later removed? If so, then it should be a 757-200M. For example, Lufthansa has removed the partition from their remaining 747-400Ms and operate them in an all-passenger configuration now. However, they are still designated 747-400Ms.
To my knowledge, Boeing never offered a Quick Change version of the 757-200, so the 757-200C does not exist as a Boeing designation.
[Edited 2016-04-09 07:31:22]