Leeana... I will have to go back and check the Rulebook but i think the cutoff for the "Under/Over" division for the Classics, Modern Pleasure, and Moderns is 43" not 42"?
The difference between Modern and Modern Pleasure is generally how extreme their motion is. Classic ponies move beautifully... they are VERY graceful and are big movers in their own right. Moderns move differently... you will see both big "open" movement but also more of a marching high movement in front (and preferrably all fours). Modern Pleasures will move up to breaking level but are not extreme. Moderns break level in front or beyond.
As to conformation, the Modern pony generally has a trademark neck that comes straight up out of their shoulders. The line from the chest up the neck to the head should be pretty much vertical. Because they had/have a lot of Hackney pony background, and Hackneys are not generally noted for a "pretty" head, the Moderns may have a heavier, straighter head than the Classic ponies which are more Welsh-like. Some people say "ugly head" but most Moderns don't always have an "ugly" head it's just different. Like the Classic ponies, the Moderns have extremely flat toplines with a very high tailset, refined bodies and legs. They generally have a longer back that they can drop for more motion in the front end. A Modern is generally built more for movement and less for looks compared to the Classic, so some conformational "attractiveness" is sacrificed for function. I would say overall type, the Modern Pleasures are a little bit "prettier" than the Moderns but not always... but sort of the prettiness level between "Classic" and "Modern."
Although people claim that a pony is "ONLY ONE TYPE AND SHOULD BE SHOWN AS SUCH" I have seen countless times people moving their pony around in type! A Classic can be shown as Modern Pleasure, particularly if they put keg shoes on them and grow their toe out a bit so they move a little more extreme. A Modern Pleasure may be shown if you put weighted shoes on them and work them enough to build the muscles to be Modern. Moderns can be bumped down to Modern Pleasure if you keep them barefoot or not work them as extremely. However, not all horses are borderline and some are pretty much exactly the division they are in and can't move up or down.
This is my two year old mare, she is Modern Pleasure:
You can see her neck comes almost straight up out of her neck, she has a heavy (but it's not ugly!) head, a longish back, flat topline, and lots of leg. She is VERY much a Modern Pleasure horse. There is no way she could show as a Classic I think. However, her motion is not very extreme, so she could never show as a Modern no matter how hard she tries.
This is my yearling colt. He is Modern/Modern Pleasure. With keg shoes on, he just breaks level. Jennifer Radoi (Sweet Opal) has his full sister that showed as a Modern Pleasure but has been bumped up to Modern this year with training and shoes. So you see, this is an example of a horse that might be borderline.
I would love to post some pictures of his sister and the difference training makes for these guys, but I may leave that up to her.
Lastly, my old 20 year mare is a Modern. She is not as "pretty" as my black mare, who is a Modern Pleasure, as this mare is built mostly to move. She has a long, upright neck with a high wither, a long dippy back, high tailset. She moves high on all fours, even when barefoot (and foundered!). I am not sure she fits the Modern Pleasure type, I think she just might be a Modern.
I am by no means an expert on ponies, but that is my take on the Moderns from what I have gathered so far!
Andrea