I like many others have done this with my 116500. I sourced my OF straps from the AD but when doing so you have to provide a serial # for a watch that already comes with it.
As you stated, if you already own a watch that already comes on the OF straps (20mm lug width), then you can supply that serial # to them with the size and the AD can order the straps for you. (the part # you would want is 218-105-xx (where XX is your size, D, E ,F etc))
I would grab the 116520 end links from smarty end links to complete the look.
Both my SS on the OF straps (last year).
