Very nice. I have lusted after a Tranter for years. Still do.
W.C. Dowell's history of Webley firearms, "The Webley Story", mentions a 295 grain lead bullet atop 27 grains of BP for the .577 Tranter no velocity given. In addition, there is a reference to a bullet of 495 grains at 725 fps using 28 grains of BP.(page 272 Table X)
A note in Dowell's book is that for the Webley model pictured, the 1st Model, solid frame, the actual chambering was for a Boxer cartridge that contained a "spherical ball". (page 61) This round ball is most probably what was referred to as the load above since a LRB of .577" diameter would weigh just about 295 grains.
Pete