Re: Locomotive Coupler Question

Couplers come in several different "sizes" and types. Standard gauge uses full size, SOME narrow gauges use 3/4 instead of full. I wasn't aware there was a 1/2 size, but it makes sense. In addition, mining operations have at least one standard design of their own (looks like a cross between a real coupler and a NMRA X2F).
MCB stands for Master Car Builders. This organization was formed for the purpose of standardizing freight car standards, parts, and repairs. It later became the ARA, and is now the AAR (Association of American Railroads).
The "E" type coupler is actually a fairly "modern" design (1930s?), and is being superceded by the "F" design. The knuckles are not interchangeable, so train crews have to rumage around to find the correct kind to carry back to replace one. The "F" also comes in shelf type (used on tank cars). In addition, couplers and knuckles can be standard (250k working capy) or HT (360k). There is also a "H" type tightlock coupler used on passenger cars. And, couplers can come in various shank lengths (12-60") depending on application and draft gear.
Before the standardization on the type "E", most of the couplers had much smaller shanks and did not often use interchangeable knuckles.
As far as different couplers for the pilot of a steam locomotive vs. the tender, yes they can be of different in shank design. Most engine pilot couplers appear to be of a very short shank, monted on a bracket bolted to the pilot beam (pivoting right behind the head). Tenders could use a more common coupler with draft gear similiar or the same as freight cars.