GER No. 180 (now masquerading as MSLR No. 15) was built by Samuel Claye (ref. MSLR article), so does that make it four companies?
I have read somewhere, the GER supplied locos and rolling stock to the LT&SR.
If so (ignoring the earlier GER no.180) I wonder if the GER designed the horsebox and provided drawings to the Birmingham Wagon and Carriage Company, the Metropolitan Railway Carriage and Wagon Company and the Swansea Wagon Company. The horse boxes thus being built in batches, with minor variations according to the builder.
Indeed, if the Gladiator kit builds into a fair representation of a near-identical GER horse box, I can decide on the company livery after the build!
I have placed four book requests with the local library. In the meantime,
the photograph of MSLR horsebox no. 9 (ex-GER no. 408) is very helpful for making a start on the model.