Trigg and Tregg?

Tregg is a manufacturer of all sorts of trailer parts, including a lock as shown in your post that WILL fit a Trig coupling pin.

Trig is a coupling design that uses a semi flexible poly block that in turn can be fitted to an ALKO coupling or a Carter Westco coupling or even a Tregg coupling.

Treg couplings

AL-KO

Hope this helps, Colin.

They are both very old, well respected family companies from Adelaide.

Treg Trailers are around the corner from me at Underdale and designed and made the "Treg" poly hitch. They are the most respected trailer-maker in SA. Started in 1946 by Gordon Tregonning who worked in the business up until retiring a couple of years ago - at 88 years of age.
http://www.tregtrailers.com.au/

Trigg Brothers are a foundry, making cast iron products such as ball couplings for trailers. They set up in 1925, and are just down the road from me at Ashford.
http://www.triggbrs.com.au/history.html

We have a 1960 model Treg trailer in the family. Some years back I needed some new wheels, which were Dodge wheels and asked at the counter whether they had any. I was led around the back to see Gordon Tregonning, who was in his 70's at the time. He took me to the far corner of a lean-to and found a stack of wheels that had sat there for 40years. Asked me whether I wanted 15" or 16". Only wanted $5 each, so I doubled his price and took them away.

Ditto re Phil's comprehensive response. I've used both poly block couplings - the Trigg is cast, while the Treg is forged - my simple mind thought that castings could not keep up with forgings re strength pound for pound, but it must not be so - the Trigg was very robust - far as I know, the currently polular, highly rated, fully articulating Hyland unit is cast too.