I'm going to guess that this means that you did win it?
That is precisely the difference between the cards. It's possible that there is some difference in the pad placement, but functionally, the only difference is the VRAM totals.
And if you had a time machine and about $5000 to spend on comparative Thunder shopping, you'd find that the Radius Thunder 24/GT is identical to the Radius Thunder IV GX 1152, except, it lacks the DSP daughter card. I had that straight from a Radius engineer, shortly before they shut their doors.
They were liquidating the 24/GT for about $400 (or was it $600?) and when "Radius Vintage" got their hands on the old stock, they started charging something like $800 for them, I think. Radius Vintage was always over-proud of their old stock.
I've always wanted to ask Macmetex what his connection to Radius Vintage was if any. He seems to have ended up with all their old stuff, but his prices have always seemed very good to me.
Anyway, the old VRAM chips are probably hard to find now. But if you get a 1600 let me know and let's get some good scans. I've got a couple of the 1360s and 1152s on hand.