They were carrying out the mod to the tank fillers. I think to reduce the orifice size and prevent filling with Jet A.
The tanks are integrel and as soon as we drained the fuel the glue went hard so we had to clean it out with the tanks partially filled. Done over a couple of hot summer days. One of the worst jobs I've ever done in a 45 year career. The refusal to pay was insult to injury.
The owner used to take it over the alps regularly and he reported a loss of power and "bits" in the drain fuel. When we took the fuel filter out it looked as if someone had covered it in snot!!!! How it ran at all I do not know.