Wow, trunnions rings is hardly a descriptive name for these things. Admittedly, the basic definition of a trunnion is a cylindrical protrusion, but it's normally associated with a pivot. Googling "trunnions rings" brings up lots of rings with trunnions on them, but not what we want.

Thinking back, my 1940 edition of the Wood Handbook published by the Forest Products Laboratory isn't any better; they call them "modern connectors."

Let's try split ring timber connectors:

http://www.portlandbolt.com/products/ot ... rings.html

These serve the same purpose as the cast iron key blocks often used in the joint between draft sills and the main center sills, they provide a shoulder to transmit the longitudinal forces, either in draft or buff.

My 1974 edition of the Wood Handbook (USDA Agriculture Handbook 72) has a discussion of types of ring connectors, and allowable loads for different types of connectors in different types of wood.

It also discusses placement of such connectors relative to the edges and ends of beams,and what effects these factors have on allowable load.

The Handbook is probably a good investment for any group doing much work restoring wooden cars. Since it is a government publication, the price is probably fairly low.