I just did the same thing on the weekend. I've got a real old Firestone keg which the LHB didn't have a poppet for. He replaced the seal and it was virtually exactly the same as yours except it was a white plastic one and the old one was a black rubber O ring. Haven't tested it yet though.
The new seal looks flat and not much like the usual seal at all - is that right? If so I imagine it mightn't fit so well. These aren't expensive items I'd try to get the usual style, just for the piece of mind. Of course if someone here has actually used that style seal then ....