Lots of verbiage here
https://books.google.com/books?id=5fxGAQAAMAAJ&pg=PA52&lpg
Scroll down to p. 32 and the 16 over C in the diamond is simply "the calibre of the chamber" without reference to chamber length, which unfortunately the illustration below calls "bore".
This calls it "gauge size"
http://shotguns.se/html/belgium.html
This "calibre"
https://mallorquina.pagesperso-orange.fr/source/page2.htm

The Belgians didn't use 'LC' for long chamber as did the British

Chamber diameter and length was optional until 1924; thereafter the gauge and chamber length in the Omega
IMHO very few pre-WWI 16g had 70mm chambers, and if so carried the length over diameter mark

[Linked Image from photos.smugmug.com]

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