The logging railroad I model used link & pin couplers up until the end of operations in 1938. Locomotives were equipped with slotted knuckles that could have a pin dropped down in them. You'll also see a variety of knuckle adapters that fit in those '3-slot' link and pin pockets that were held in place by the pin, that was a typical style that Climax used. This shows you just how nuts it could get. This was Lima 2-6-0 #5, you have a two-level drawbar because the log cars were so much lower than normal, and a regular knuckle, and a slotted knuckle. So retrofitting those old 4-4-0's for knuckles took all manner of creative thinking. But most had the 'cowcatcher' rebuilt much shorter in some way, or entirely removed and replaced with a switcher pilot. There certainly was no standard here. Here's a good shot of the 'three holer' on the front of a Class-A Climax: http://www.climaxlocomotives.com/history/img/?P=10 Climax sold log cars with 18" wheels, so with those cars, your coupler pocket is going to be a lot lower than normal. My prototypes cars are incredibly low.
I may have missed the listing in Shapeways for the 3 slot type knuckle for link and pin and the pilot rod. I saw the ones that are somewhat similar to MTL offering on their Civil War era cars.
