Quote "Marshall Aerospace is the Sister Design Authority for the Royal Air Force fleet of Lockheed L10-11 TriStar Tanker/Freighter aircraft which Marshall converted to these roles."
Which means Marshall has "Design Authority" over the converted parts of the tanker/freighters, not entire airframe.
I worked with Mashall for 20 plus years on the TriStar. If fact when I retired, Marshall and the RAF, each gave me a retirement gift. Which is more than I can say for Lockheed.