The DC-10-15 was a DC-10-10 with DC-10-30 engines.
Fred Laker came up with the wheeze of using his DC-10-30s on short routes with the centre gear retracted in the hope that the landing fees would be based on the DC-10-10 landing weights.
It didn't work though for everyone charged on the max landing weights for the individual aircraft concerned; ie DC-10-30 landing weights (403,000 lbs if my memory serves me right).
The other problem was that some crews forgot to lower the leg before landing back at LGW and this used to p*ss the ground crew off something rotten for they had to jack the aircraft up to get the centre gear down and locked.
After a short period we forgot all about the idea of isolating the centre leg.
However, I remember later in life when working for a Far East company that we had a deflated tyre one night at Seeb. We were able to pinch one off the centre gear and fly back to base as a DC-10-10!