Dave
Edit - Since typing this reply to your post, I've discovered some later posts addressing your observations, but rather than deleting my post, I thought I'd leave it in for you to take a look at the link I've included.
The wagons you mention are two completely different diagram numbers. The commonest type with the deeper lip is the good old diagram 1/108 (welded construction), whilst the one with the thinner lip could well be a diagram 1/109, of rivetted construction, with pressed end doors. In the photo of the class 37 you refer to, it looks like the first two wagons are a diagram 1/108 and 1/109.
In 7mm scale, Lionheart trains make both versions, as outlined here....
http://lionhearttrai...ge=models&mid=2
I'm a bit out of touch with 4mm scale, and though I know the 1/108 (welded) is available both as rtr and as a kit, it's possible that the 1/109 (rivetted), is available too. You mentioned Farish in your post, but I'm afraid I know even less about 2mm scale!
Hope that helps
Dan