HKS 45 colt speed loaders is as close as ive found.
But the 455 rim Dia is a tad wider than a 45 long colt and to tight for 455 Webley cases to fit into that loader.
+ even the Domion 455 cases are fairly short for this speedloader.
BUT what i have done is Bought 45 Scofeild starline Cases and shorten there length by about 1/8th and thin the Rim from the forward side to 455 rim thickness and load 250 gr .454 dia keiths Bullets.
Now you have a longer case with a shorter bullet and the speedloaders work great.
It turns the MKVI into a much harder hitting revolver as you can load up the solid head Starline Cases quite hot.
Note: Still becarefull not to load to hot in the old guns but its much Better than 455 ammo.
Seems like alot of dicking around but once the cases are done they load with the same 455 dies ECT and the cases last a long time.