I'll try to answer and correct all the misinformation here in one not so simple post;
Gasket material can be added, but should not be necessary. Putting a piece of flat material in the channel grove and closing the doors will show if it's too tight. New hinge pins are usually the answer to a correct seal, and the easiest fix. (Actually greasing the original pins yearly is the easiest fix) A dollar bill closed in the doors should pull out evenly all the way around with the same resistance. (cold, or it will cost you a few dollars to test) Doors can also be lapped with course grinding compound by laying stove on it's back, remove hinge pins and work door in circular motion with door weight to create a perfect seal between door and frame. Any cement from previous gasket material should be removed. People only think a gasket is needed due to all other stoves they've seen using them. As per Fisher Stove Works; Doors may be "surfaced at seals" removing no more than .050". Meaning some may be machined smooth for a better seal. The rough cast edge can be lapped as described above removing no more than .050 of material. Normal amount of lapping is only going to remove a few thousandths on both the door and channel to make a perfect match.
All Fisher Stoves ARE airtight in design, patented and tested as the first "airtight" stove made.
Chimney draft creates the negative pressure in the stove, pulling the doors tighter. A backpuff, or rapid combustion creating a positive pressure can push the door outward, leaking around the seal, but normal use is considered air-tight as designed.
The purpose of the "Smoke Shelf Baffle" plate is to roll the smoke back into the flame. EPA testing showed particulate without baffle 60 grams per every kg. wood burned. With baffle (Grandma III) 6 grams per every kg. wood burned. 90% reduction in smoke. This was acceptable for new standards that took effect July 7, 1986, but not for standards set to take effect July 7, 1988. (when they stopped production along with most others) Below is a picture of factory baffle, they sit on 1 1/2" angle iron, they are NOT to be welded in place due to expansion (lengthwise growth) bulging sides.
Second purpose is to direct most intense heat in stove to the stove top. Without baffle, this heat radiates rearward into elbow of rear vented stove. The intense heat at the slot created is the last chance to burn any smoke particles that would go up the stack. (Oxygen introduced here would have created the secondary burn and many companies would still be in business. The fire oxygen depleted this area, so particles went unburned)
The reason you find different baffles and boxes is due to revisions in prints sent to fabricators on the following dates; No baffle was used in the first design in 1976. This stove can be identified by "76 Star" doors that use a 1/2" round rod and round groove cast in the doors. No gasket material can be used in this design. 1977 dropped the Stars and went to a regular square groove in door with 1 inch channel iron 3 contact seal. June 26 1977 a "Draft box Assembly" was added. This is a fabricated box over the outlet, probably what noodles is referring to an "angle iron fixture". The revised print of the parts shows size and part numbers if you need to verify if this is what you have. I do not publish pictures of the prints.
July 22 1977 a "Draft Box Baffle" was added. This changed the model of the GM and GP to a GM II and GP II.
1980 was the big change to round top doors and a "Smoke Shelf Baffle". (GM and GP III) That is what you're referring to by building your own internal baffle across the stove. Pictured below, they are made of the same material as the top, 5/16 HRS.
poconoeagle referred to "boiler plate steel" construction. This is false. Boiler plate is certified, very expensive steel for use in high pressure boilers. All material used in manufacture of a Fisher Stove is designated to be HRS, Hot Rolled Steel. (you may know me by the way, I'm retired now but worked as a steam mechanic at Steamtown when the 2317 was put into service, and rebuilt the Baldwin 26 yard engine)
The size for a homemade baffle should allow the same square inch area that the exhaust vent is. Trial and error is one way, here's the mathematical way; Fireplace Series double door stoves are made in 8 and 10 inch outlet. "Restaurant Model" stoves called the XL are 10 inch, and 6 inch "Bear Series" single door stoves, you need to figure outlet sq. in. by; radius squared X pi
(8 inch pipe = r of 4 X 4=16 X 3.14 = 50.24 square inches) This would be the opening above baffle. This should also direct flames toward upper top radius. About a 45* angle. Different stoves call for different angles due to box size changes from 4 piece box pre 1980 to one piece box, post 1980.
The easiest way to install a baffle plate is simply setting on two bricks, set upright along the sides on top of the first course. A-36 mild steel plate does not warp or sag. Depending on steel prices, about $30.00. It's the single best improvment to make to any Fisher that doesn't have a baffle. I would not use one without. Pictures and instructions here, http://www.hearth.com/talk/threads/74710/
I don't like to refrence different websites, but most of my work is published there with the most Fisher information available online. Everything covered above is referenced on the "Everything Fisher" thread I authored almost 2 years ago. (except the dates for revisions on prints above)
Here's the thread detailing the Grandpa Bear. My intention is to detail each model of the Fisher Stove; http://www.hearth.com/talk/threads/69448/