Original fit codes were:
K = Clean
L = 2 x 2250L
M= 2 x 2250L + 2 x 1500L ferry fit.
IIRC, when they did the figures for the Cyprus transit the fuel figures, due to routing as the weight meant they were unable to reach the required airways minimum FLs for the most direct route; the ideal fit was 2 x 2250L + 1 x 1500L; but this had never been investigated or cleared and the money wasn't available to trial the fit.
The dearth of tankers meant that the IOW top-up was the only option for some planned dets, the other option was to stay at home.