I think with "GB (U.S. Army - used), 1944" they try to indicate it is of British manufacture, and was used by the US Army - which could have been the case in the UK I think, where the Americans also used K2 ambulances and other British vehicles on airfields and other bases.
My guess is that this trailer was acquired for the museum collection a long time after the war and that there is no connection with the BEF and Bastogne. The "(U.S. Army - used)" is only there to try to "justify" why it is on display at the museum?
For example, their Sherman M4A1(76)W HVSS displayed outside is an ex-MDAP Sherman delivered to the Netherlands Army after WW2, which was recovered from a range somewhere in the 1980s and then donated to the museum.