The main difference seems to be the lack of antimacassars, which are meant to be there (as you can see from the first photo) but seem to go missing with these seats.
Hopefully they've got someone trying to solve that, or design some other way of marking the first class seats.
FCC did use totally different seat coverings on the 319s, 321s, 365s and 317s but not the 377s. It does seem that these are perhaps the hardest trains to spot first class, regardless of operates them. They are of course designed to Southern's specifications.
Different seats would certainly help, although usually having a lot more legroom or perhaps 2+1 seating makes it even more obvious, as well as carpet on the floor etc.
In the case of the Thameslink 377s the antimacassas and labels all appeared to have been intentionally removed shortly after the franchise change. Presumably because they had first capital connect (or southern) written on them. New ones appeared in the last week or so.
I can't remember what train it was, but I've seen on at least one train coming back from Brighton that the antimacassars didn't have anything to attach themselves to the top of the seat (usually two holes that fit over two small poles) and relied on the sticky tape - but whoever put them on never peeled off the tape over the sticky section and had just tucked them in at the top - or tried to.
They can't have been put on a long time before, but as soon as we went through a tunnel with the windows open, loads of them blew off!
These were plain white ones with a grey circle and white '1' that had replaced the previous First Capital Connect marked ones - and they might not have been the right design/shape for the trains. FCC was still operating the trains then, but had presumably been working ahead of time for the change not that long after.
I had a similar thread, with some photos (e.g.
http://www.railforums.co.uk/showpost.php?p=1922670&postcount=9 ), showing how GTR had removed all the original first class signs and stickers and put up a single A5 sign that wasn't that clear - on a door that could be left in the open position and easily missed.
All the seats had the '1' but I am not sure that a light grey on white is that good, and if there were lots of people sitting and blocking them then it's hardly sufficient to rely solely on these to tell people.
I assume things have improved now, and I've spotted a few trains where they've got stickers on windows and the main doors.
Funniest thing was seeing a refreshed 365 (having FIRST & CLASS written on each door in white text on a light blue band/ribbon) where FIRST was missing. I never twigged at the time that someone had obviously removed the left hand sticker because they'd been told to remove anything saying 'First'.
