GOT IT!
The new (current) border crossing is on the 2201 which is clearly within Surin province. The former border was east of the 2201. Where the 2201 road bends to the right just before the border crossing there is a left turn, along which the Surin Sisaket provincial boundary runs. This was the road one took 5 years ago to the old border crossing. Go to the end then turn right and the THAI immigration office was some 200 metres along on the left. The Cambodian Immigration office was diagonally opposite. The road on which the 2 immigration offices stood was also the provincial boundary. Close inspection of the map will show that Thai Immi on the left was in Sisaket province. as the provincial boundary (the dotted line) appears to run down the middle of the raod.
So having established the old border crossing was in Sisaket province, and known as the Sisaket crossing, it has now moved a few hundred metres west, which whilst firmly planting it in Surin province, will in all likliehood still be referred to as the Sisaket crossing, and is conceivably still controlled by Sisaket immigration.,