You are correct about patent use numbers. They indicate authorized licensed use of a patent for which a fee was paid. When patent protection expired, PUNs were no longer necessary. I believe UK patents also required an annual fee to be paid to renew protection, and if this lapsed, protection and PUNs would end. How to find out when renewals stopped for this patent is something I couldn't find. So even at 20 year's duration, you know production should be no later than 1892.