I think there's a difference in body lines in the CMC/FF/SB/early IM cars that nobody else has, too -- I may be incorrect, but it seems to me that every CMC (et al) I've seen have that flattened arch on the rear wheel-well opening. In addition, the CMCs and their ilk had about four hundred gauges in the dash, reading 'Classic Reproduction' or something on them.
Another, subtle, difference can be found in the tail lights. Some of them, maybe just a couple Becks lately, have the lens in upside-down. I am not sure if that's a "thing" they're doing at the factory -- or maybe it's a suggestion from someone here. I've flipped mine over since I first saw it done.
If you were to sit inside a Beck, you might also find that the seated position puts you up a little higher off the floor of the car. In a VW-based car, that height is reduced a bit. In mine, which is a complete mutt, I'm almost two inches lower than in the Beck Speedster -- I can't rest my arm on the door tops.
I'm probably not the best basis of comparison. My car's a ham-fisted wreck, compared to most, and my upholsterer was the silver guy in the Wizard of Oz.
You could come to one of the events this year, say ... Carlisle? ... and have a look at all of them, side-by-side. Just a thought?