Choke 17.5mm = .689", 18.3mm .713""
17.5/25.4=.689" ;; 18.3/25.4=.720", might want to double check your math on that 18.3 dimension.
Choke on Belgian guns was marked by several methods over the years, but generally when two dia's were given as a fraction as appears on this one they indicate both bore & choke dia's. I would thus suspect the bbl marked 17.5/18.3 had a .720" bore dia & a .689" choke dia or .031" of choke constriction. If only one size is given along with the word choke it shows bore size & simply shows the bbl was choked, but not to what extent. I believe though in all cases the bore dia was given as this was the important dimension as regardss to proof.
The 89.5 grains of powder would have been Black Powder & equals to about 3¼ drams, so the gun was proofed for a 3¼-1¼ load @ then current pressure levels, as stated not todays SAAMI standard. "IF" in proper condition the load you mention should fall within those parameters.
