This ruler was originally handed down to an aunt on my (English) mother's side of the family. One side is a standard ruler in inches but, curiously, the other side is unlike anything I've been able to find on Google images.
One side has inches. Many such such rulers can be found on the internet.
The other side seems to be for prices (lettering includes L S D, presumably pounds, shillings, pence).
This close up shows D and L on one section, then S and D on the other.
Resolution and contrast are not ideal. The faint numbers in the 'S' column between the very clear '17' and '11' on the upper half appear to be 19-2-4-6-9.
The wooden folding ruler also shown here is a Rabone Boxwood N°1380 (of no particular value or interest I think).
The earliest known owner was a great aunt (born 1876, died 1941); we don't know who (if anyone) owned it before her or how she obtained it. Nor do we know anything about her, other than that she had several brothers and that she never married. This great aunt's father (b.1831, d.1904) was a member of FRIBA (Fellow of the Royal Institute of British Architects).
The ruler is ivory and has, in tiny letters on one side, the name Joseph Mackness. It is unclear if this the manufacturer (Googling hasn't turned up any manufacturers of this name) or the original owner. There are some people by the Joseph Mackness on Ancestry.com but none seem to fit the likely time-frame (Victorian or Edwarding), and there are no known Macknesses in the family.
Given the numerous examples of ivory and bone folding 'rulers' that can be found online, many for different purposes / professions, it is puzzling that I can't find any examples with D and LSD. This might indicate that it is quite rare, which seems strange if its function was related to prices; (presumably) many thousands of shopkeepers, customs officers etc. would have found it useful, which means it shouldn't be rare.
One explanation suggested by a family member is that it is a presentation item, souvenir or retirement gift, never intended for actual use.
Was this folding 'ruler' used by shopkeepers, customs officers or some other profession? Or is it a presentation item or souvenir?
I'd also be interested if anyone could put a narrower date range on it than (I think) Victorian to Edwardian.