Well, while I'm working to try to reduce my image sizes, I can send photos via text or email.

A little tough to say what it is. A #6 lever, but that's kinda where the #6 ends on it. The #6 usually had a flat side heavy receiver, and yours is stepped side. The Union Hill had the rebated or stepped side receiver, and a pistol grip stock like the spare stock is. Both had pistol grip receivers, but the stocks were sans a pistol grip on the #6, similar to the aftermarket stock on the gun, except heavier stocks.
Values can run all over the place, and it's tough to set a value on yours. If the barrel on it matches the receiver serial number, it adds value IF it's a great bore. If the spare stock has a matching serial number on the end where it butts the receiver, that's a plus too. If the spare barrel is a great one and both barrels are great bores, it adds value. Would be better to see it in my hands to look for markings on either barrel, although the spare barrel appears to be a very recent addition in the way it's built.
My loose "guesstimate" without seeing it close up would be $1500-$2600. Couldn't get any closer from just pictures.

Just noticed I included a photo of a rolling block. Please disregard.

I agree, BUT, that's the time element.  Most buyers of something like this would be interested in the action and 1 butt stock, hoping to sell off the other parts to recoup some of what it cost to build it.  It's certainly worth more than the sum of it's parts.   
If the seller wishes to maximize his return, pull it apart and strip the plating from the action and sell the action, stocks barrels palm rest all separately.

830singleshot wrote on Apr 20^th, 2021 at 5:16pm:

Can't imagine how time would factor in? Might take all of 15 minutes to break it down. I wouldn't care about or try stripping the receiver. I'd use it. 
Yes, whatever wasn't used would either get sold, or put into my future projects box. Still think it's worth more than $1,000-$1200, but that's just my opinion.

I don't think Marlin guy isn't far off, Its a shooters gun and a little tough to estimate when the work was done. if you screwed a pistol in my ear id say 1950s with the 357 barrel 1990s. its not my taste and a lot rides on the condition of the Winchester barrel but when I looked at it i estimated $1,800 to $2,500. It might go a bit more if the molds and tools for the gun were with it.

40 Rod

Jack I'm once again confused? How does the buttplate fit both stocks when the spare stock appears to have inletting for a long return on the top of the butt? It looks more like a Winchester Helm inletted stock than the style that's on the gun's stock.