The JPG that Stavka has the link to in his post is not a Webley, but a Colt Detective Special, which from the looks of things in the basement scene, is indeed the pistol Clemenza gives Michael. The gun has some distinctive features wich give it away, even in the brief shots in which you see it in the film.
First, the ejector rod under the barrel is completely exposed, with no lug under the barrel to protect the end of the rod (as on a Smith & Wesson Chief's Special), nor a full shroud that the rod fits into when the cylinder is shut (as seen on Ruger revolvers and later versions of the Colt).
Secondly, the cylinder looks like it rotates counter-clockwise when Michael pulls the trigger- the counter-clockwise rotation is a dead giveaway that it's a Colt, since the other two major American revolver maufacturers of the time (S&W and Ruger) both had clockwise rotating cylinders.
It looks like what Michael has is a Series 1 Colt Detective's Special, chambered for .38 Special. This revolver was first introduced in 1927 as a smaller, more concealable version of the very popular Colt Police Positive .38 (which was in and of itself a version of the Colt .45 Revolver used in WWI and carried on at least two occassions by Indiana Jones). The original butt is indeed squared off, as is obviously seen even with the tape in the film and in Stavka's link. However, it looks to me like the grips in the film aren't original, as they're much broader at the top with a sort of thumb-rest built in. This was apparently a common modification made to this pistol because the size of the gap between the front of the grip and the rear of the trigger guard was quite deep and wide, and was uncomfortable for shooters with small hands. This is why the sort of larger, beefier grip that Michael's gun has is basically the standard for revolvers of this size in modern times.

Sammy the Fish