James, there was at least some .455 ammo imported, I have the old ads (1950s vintage) with both the gun and ammo for sale. Been a while since I looked at one, but I know I have them somewhere in the slowly mouldering stacks of 50+year old magazings cluttering my cave...
Can't be certain till I find one and see, but something is telling me the ammo was going for $1.70 a box...
While there was never a supply in the US like the .45, or even the 9mm, there was a quantity of .455 imported, along with the first guns, I think. I think when that batch of ammo ran out, there wasn't any more to be had, or not for some time, at least.
Scarce ammo supply, plus the strong desire to sell the guns, and a somewhat different attitude towards safety in general than we have today is what got so many Webleys converted. Also, in those days there was NO hot .45 ammo, it was all ball or target loads, and after all, Webleys rarely blow up with ball .45 ammo, so, why not? Right?
Different era, different attitudes, and maybe some folk didn't know, or didn't care about things we consider important today.
All else being equal (and it almost never is) bigger bullets tend to work better.