The shortened barrel and markings including Birmingham 4 are correct for a Junior in the J30*** range. I know of several and my own example also has a gloss black finish, which I've never been entirely convinced is original. The highest Junior serial numbers known to me are in the J34*** range but the weird thing is none are known between the mid J30***s and J34***.
Webley released a number of 'Anniversary' models to celebrate their 150th Anniversary in 1940 and marked the boxes by adding a sticker. I also have a boxed Mark 1 with such a sticker and have yet to find a boxed Senior with the sticker, albeit I am satisfied the latter do exist.
Kind regards,
John M
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.