Looks like an early "roller bearing" truck

The sort where there are plastic caps that spin on the end of the wheelsets. 

Aside from that, no idea.

Roller bearing

Its designed to caps put on the axles ends so that as it rolls down the track the bearings rotate like a real car.  I remember them being offered but can't remember who the manufacturer was.

Kato Barber S2 truck

The 2002 Walthers catalog shows a truck like that with spinning wheel caps sold by KATO as a "Barber S-2" truck.

Kato

Looks to be a Kato product.  The current Kato truck is part# 31-602 

I buy these whenever they come up on eBay as they roll very well and the rotating bearing caps look cool in a runby.  Barber S2 70 ton trucks.

So Nelson am I to assume they

So Nelson am I to assume they are no longer available as new stock?

Are they limited to one wheel size axles?

Barber S-2-A

That's definitely Kato's take on the 70-ton Barber "S-2-A" configuration.  The Kato logo is visible in the first photo, cast onto the underside of the bolster.

Tangent currently offers two different versions of this prototype, one with rotating bearing caps, and the other without  https://www.tangentscalemodels.com/product-category/trucks/ .  They also have semi-scale code 88 wheelsets and standard code 110. 

Since this is a 70-ton design, it would be used with 33-inch wheelsets.

@Brian

According to thier website they are still a current offfering.

https://www.katousa.com/HO/accessories.html

I tried to order some, but

I tried to order some, but the first link said 'out of stock'

The second link said coming soon, but didn't allow one to place an order??