I have available reproductions of the "Lachenal & Co. Patent No. 4752. London." straps that were on some Lachenal McCann Duet concertinas made in the 1890s. The leather is 1.5mm and has been carefully selected to match the original. The gold blocking is done with modern gold blocking foil (not gold leaf). Price is US$35 (24 pounds Sterling) for a pair, inclusive of airmail postage worldwide.
You can email me on firstname.lastname@example.org to order. Payment by PayPal. I will make a contribution to this site on sales.