Chances are that when the summer season gets into full swing (from May) GSM will have leased another 767 to cover so May - Oct will more than likely have the 767 on the SFB routes. They may still throw in the 75 now and again but not unless they have to.
The problems just now are due to them reducing the flights and A/C size because of bums on seats over the winter. Add to that the ETOPS mess and its not something they can sustain when number of flights increase from March.