Just to add, it is quite legal to shorten the barrel of an antique revolver so if you were to buy an extra upper for a webley, you could shorten the barrel and swap them at will. You could even do this to an original barrel though that would affect the collector value negatvely.
A mk VI upper will fit a mkI** or mkII* without modification and (this I have not tried but sounds reasonable ) a mkI or II with minor modifications to the upper.