Biff did an excellent job at answering that question. And to add a bit more light to the subject, I did a research project on the shock frame chassis stamping numbers a few years back.
All Bugeyes only had the Thompsons body pressings number under the wire loom on the right side.
And since Healey started the Bugeyes with chassis # 501 and Thompsons started with 001, there is indeed a 500 number difference. Not exactly 500 mind you, there was no rhymn nor reason to what chassis # was fitted to which body shell.
Things like when 3 Loorys left the plant loaded with tubs, and 1 driver stopped for lunch, the other driver had a flat, and the 3rd driver made it to the factory before the first 2, the numbers were throughly mixed up. Also if tub # 1001 went on the truck first and tub #1008 went on last, #1008 came off first and got chassis number 15001.
So the numbers are close but of no importance.
Something like that was said by Geoff Healey in 1992. So if anybody finds a Bugeye with "matching numbers" it's a fake.
MKII Sprites and up all had the chassis ID # stamped on the right front shock mount frame.
I personally checked into each model except HAN6.
I did see it on the HAN7 and each model afterwards. I have been told it is on the HAN6.