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Hello "TMT", welcome to the forums.

I think you likely know that there will be few and far between owners of a vintage Explora. Thus getting any load help specifically for an Explora will be tough.
There are several owner/shooters besides myself on here that do shoot vintage bore rifles or "Paradox type" guns that may be of some help.

That said, I would look up any older posts by Cal Pappas (since deceased) here or on that "other website", as he was somewhat of a "bore gun/rifle" guru.
As the period 12 bore Explora used a ~730 grain bullet I would investigate a similar weight bullet mould of the WR or Fosbery type shape. For a "Paradox/Explora" type gun you best slug your bore first so as to get a mould that casts about .002'-.003" under actual bore diameter at 9" in front of the chambers.

Likely the gun was made for brass case ammo? "Rocky Mountain Cartridge" is your go to source. (do accurate chamber casts)
Cal Pappas had best results using Blue Dot powder for his smokeless/Nitro regulating loads.

A whole lot of individual research for you to do as liability concerns inhibit recommending loads to anyone.

Supposing you could contact Westley Richards for the history on your gun and they maybe can recommend what load to try or at least clue you in to what the original loads were?
I have a .400 WR and they are very happy to provide history details.

- Mike