If, as you say, it is one of the early NE rather than BPE rifles, then you have a very useful DG rifle. Graham has a very comprehensive book on the history of Greener and its guns.
https://www.abebooks.com/servlet/Se...ome-_-Results&an=graham+greener&tn=&kn=&isbn= Not the cheapest, but now that you have joined the Greener Society, not a bad addition to the library. The Sovereign was one of Greener's strongest actions early in the nitro era, hence its use on a big double rifle. The rifle would have originally been regulated for a cordite loading. Assuming the barrels are still in good shape, and the solder still solid, I am sure you can work up a load which will regulate well. Greener's rifles were not Holland & Holland Royals, but were extremely well made "working" rifles. As you note, they are rather scarce. They made large quantities of export shotguns during this period which generated far better margin than bespoke rifles. Would love to see some photos of your new acquisition. If you haven't, you really should get that rifle to a competent double rifle expert, and have him go over it very carefully (depending upon where you purchased it, that may have already occurred?) 100+ year-old rifles and guns have a way of attracting the attention of shade tree gunsmiths with no clue what they are doing with a double rifle. Better to get things right now than discover it at the range or on safari.