Don't mean to stick my oar in here but for what it's worth, I have a 1903 MS take down rifle, also with British proof marks, which has exactly the same set-back fore sight as your rifle.

My rifle doesn't have a date of manufacture on it either. However, for various reasons I believe it was probably manufactured somewhere between 1920 and 1925.

Those reasons (which may apply to your rifle but then again maybe not) are:

1) It has a capital 'T' in a circle on the barrel. This mark was apparently put on rifles during or immediately after the First World War.

2) MS sporting rifles weren't made during WW 1, or for a few years thereafter; and

3) Rifles that were imported into the UK were required to be so marked from 1925. My rifle doesn't carry such a mark.

When exactly the set-back fore sight was introduced on MS rifles, I don't know.

Anyway, hope some of this may help.