The Mk.1 was cheaper than the Senior, and possibly didn't get the same level of care as the costlier Senior.
I've never been a great fan of the Mk.1 even though I've still got a couple in their boxes.
My oldest childhood friend died last year, but the last time we spoke he promised me his boxed Webley Mk.1 if I outlived him, but when I spoke to one of his sons, I told him to give it to one of the grand children.
Like said, Seniors seem a bit more common nowadays than the Webley mk1 pistol. I have downsized my 12/14 senior collection over the years ,but still have some lovely boxed examples left , seniors being my all time favourite pistol in the Webley range. Ido have two nice boxed mk1 pistols in collection and a refinished one so that makes 3.