Those Peter-Bilt Air-Trac suspension cut-offs are pretty popular in the OTR custom market for reasons a bit different than the MV market.
The Air-Trac is what you have pictured, the Air-Leaf supports the airbag from a z-shaped leaf spring, and the Flex-Air uses a bid curved front facing leaf, also used bt Kenworth (Paccar) ! Also one called the Low Air-Leaf. The one you have shown from your friend is by FAR the preferred choice, in my opinion ! Here in Joplin, 4-States can barely buy wrecked donors fast enough to keep up !
The main selling point is the low profile of the ride from the top of the frame rail, in essence lowering the fifth wheel , and then raising the plate to allow the installation of custom fenders, IE, fiberglass or stainless full fenders, and keeping the trailer under 13'6''!
As sought after as they are, they are plentiful and have proven to be very durable and strong, on road ! Not sure how it would handle any serious twisting and flexing, in addition to being able to mount the top loaders ?
Will be very interested to see how this works out, despite the fact that when I saw the pictures my reaction was " NO ! , I've been trying to get away from 379/389 trucks ! "
P.S. Not to mention, you could build two Bobbed Deuces out of 1 tandem cut-off !
Do keep us posted !