I do not have deep knowledge of the time consequencies concerning the truck production and Lend Lease deliveries to UK, so I am not able to argue with above mentioned. But here is a picture from Studebaker manual. The short bumperetes are clear. I can not imagine the reason why the poor mechanic or whoever else would change the bumperetes on GMC for those from Studebaker. If it got damaged they would simply remove it (if the rest of the frame would be OK). I believe the simplest explanation use to be the right one
so my guess is the vehicle is Studebaker. But I would be much obliged to any comments concerning the census number.
And a small note at the end, the picutre from the russian web Autogallery which Bill is speaking about, is not the authentic one, it is picutre of restored vehicle, so although it has long bumperetes it does not mean that it used to be original feature.