1. This a duplex proof rule.
2. It is made by Dring and Fage, a name frequently found in the UK. Their address is given as 145 Strand, London. They were at this address from 1883 to 1902  but were in business from 1790 to 1940 (P Hopp, p 66).
3. The rule carries the patent number 14623 but this was made at a period when British patents started from 1 every year so that it is of little help for dating the rule.
4. On the side of the rule is the maker's trade mark.
5. The rule also carries the number 173 on the side. It might  be a series number as it is difficult to interpret it as date compatible with those suggested above.