I found this picture of 4-6-0 SLSF 748 on Mike Condren's website. The drivers look different to me, like they have wedge shaped balancers(?). I do not remember seeing anything like them. I thought the attribution would attach to the photograph, but it did not. It is a Jack Avery photograph from the Louis A. Marre Collection.
The triangular counterbalances were an early version of attempts to balance the forces involved in the reciprocating and rotational masses of the rods and drivers. As more was understood and machining and casting sophistication grew, the types and shapes of counterbalances evolved.
I have read several period engineering articles, which admit that the physics of counter-balancing and cross counter-balancing locomotive drivers to be a poorly understood science.
I would have to look a bit further to verify, but I think it is on a Frisco 4-6-0 where the drivers’ counter-balancers are disc-shaped, creating a distinctive Mickey Mouse silhouette effect. Frustrations for the era-specific Motive Power folks aside, these details sure make modeling the prototype great fun.
