WOW an 8 inch protrusion down is the secret. I can't tell if you have a manual damper in the flue, but that would help even more to control draft mostly during open burning, and fine tune it closed. Is the outlet 8 inch reduced to 6, or 10 reduced to 8? Double wall pipe that high to prevent rapid cooling of single wall pipe? Reducing size is great for heat output, but like I mentioned, codes are written to only allow an increase in flue size, not a reduction. The stoves with double doors back then were sized with an oversize outlet for open door burning, so closed it becomes a radiant heater and you're just right, reduced. Unfortunately the codes don't take that into consideration. If an insurance co. or code official comes out to look at it, (as long as they're ok using codes or standards for an Unlisted (untested) stove, they will be looking for 3 screws in each pipe joint, floor protection 18" all the way around, (shown in a diagram here;
https://www.hearth.com/econtent/index.php/articles/wood_stove_clearances_installing_it_safely )
One of the things that would have needed to be changed on the stove for UL approval would be a frame around the screen to prevent logs from rolling out. Cool spring type handles, so the part you touch when removing it stays below certain temps...... that sort of thing.
The 1980 ad mentions the bafffle, so if yours has none, (a plate across the stove below the outlet to prevent heat from directly escaping on an angle that also creates turbulence and rolls unburned particles back into the flame) it would be earlier than that. If they progressed like Fisher had to, and keep up with particulate emissions, they would have had to go to the baffle, and possibly bent corners instead of welded seams in the firebox corners for UL and other testing approval. When one was approved, it was used in the advertisment as a selling point, so dated ads help date the improvments and UL Listing dates.
For efficiency, think of the outlet being a vacuum pulling the heat and byproducts of combustion out. You don't want this suction at the top where it naturally rises and escapes. You want it low, like they did with the flue extension into the box. The longer you can keep the heated gasses in the stove, circulating with turbulence preferred from the step stop, or baffle, the better. So slowing down the velocity at the collar with a damper is the idea. The higher the velocity, the lower the pressure. Barometric air pressure rushing in the intakes is what is trying to balance this low pressure area. So less pressure differential from inside to out, decreases the amount of available air, slowing the burn. In the large area it's in, you may already have the balance you need without the damper.
If you're close to the border, I'd take a ride to the shop between St Paul and Minneapolis to see if Peter is still involved with the business, if it was sold, or maybe being operated by someone who worked there when they were made. (the only Peter Solac in the country is still in Minnesota and 62, so he may still be the owner there, doesn't hurt to ask!) I've found most Fisher fabricators had workers at the time that became managers and now owners of the hearth shop, so they can give you the history and details and stories of the stoves development.
This guy may have copied at the time from J & J Fabricators (Jake Jackson) in Oregon who came up with the double door stove idea and started Frontier, so he could have stolen it from them, not Fisher....... but then he may not be forthcoming with some information if he feels guilty about it today. It's been so long ago, most I've talked to have let it go and we can laugh about it today. Here's Jakes double door patent; (broken link removed to http://www.google.com/patents?id=eMotAAAAEBAJ&printsec=abstract&zoom=4#v=onepage&q&f;=false) His address was only a few miles from Bob, and he actually went to Bob to have him weld an outdoor BBQ for him and Bob turned him down since he was busy with his new stove shop. Instead, he taught Jake to weld and sold him a license to build Fisher Stoves. He even loaned him $1000 for his first steel order ! Jake wanted Bob to start a 2 door line so people could view the fire, Bob wanted no part of more door patterns. So Jake started making his own Fisher version, got caught in his shop in Redmond OR and agreed to terminate his license.
The door sealing system may show which one he copied from. Jake used 1/2" round rod, Bob Fisher used 1 inch flat channel iron. Neither used gasket material. Wish I was close ! Good luck !