I have a Plomb gear puller that belonged to my Grandfather. I inherited  it in 76 when he passed. It originally had a wood box and a single end, 9/16" open end wrench. One day a long time ago someone walked off with the wrench and box when I wasn't looking. I will make a new box for it, but need a wrench. Even if it is not a Plomb, or it is and double ended, that I can cut off. needs to be about 6" long with handle and open end.

The Proto ( formerly Plomb) set sold in September is very like the one some so-called person stole from you.

https://www.ebay.com/itm/Vintage-Proto-4011-Gear-Puller-Straight-Jaw-Puller-With-4015-and-4016-arms-/183400708060?hash=item2ab388b7dc%3Ag%3AQx4AAOSwtltbgze9&nma=true&si=mKUlcmIRLkgir1Qb%252BqmrHelxr2k%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557

They didn't get the puller, it was in use on an engine. The thief just took the wrench and box. The wrench fits in the box with the puller and being single ended you can use a pipe on it for greater pull. I would love to find an original wrench but will settle for any  old wrench to match fit and finish with my set.

Here you go.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=williams+engineers+wrench+9%2F16&_sacat=0

How about these?
https://www.ebay.com/itm/Plomb-Single-End-Wrenches-2-pc-3514-7-16-3518-9-16-V-PLVMB-V-GD-PL7-6-18/401563963482?hash=item5d7f13d05a:g:BusAAOSwdA5bP-Mi:sc:ShippingMethodStandard!94706!US!-1
-Don

Might consider a "set screw" wrench.
Single end, closed end, square hole.

Williams calls it a "square-box wrench" for set screws.