Series III (small glass) and Series IV (large glass) had factory baffles. They were laid on welded angle iron and easily removable. They are not welded in place. But they were designed to be able to heat a flue 8 inches or larger in case anyone hooked them to existing fireplace chimneys. You can now adjust it for your chimney for a more optimum burn. The object is making the smoke space (the area the exhaust travels through) the same size as your square inch area of chimney flue.
Make a template with a piece of cardboard.
Set it on the brick retainer shelf at rear.
Angle upward toward front so it comes just ahead of the lower bend in top.
The square inch area of your flue is 50.24 square inches. Measure across the stove inside. I think it will be 27 inches. (that is the magic number to fit bricks and was the depth of a Mama Bear as well, and both would take a 24 inch log) That gives you about 1.86 inch slot all the way across to equal the same square inch area.
If you weld angle iron on the sides with a leg facing down, you can set a brick on each side on edge, and set the baffle plate on those bricks. The angle iron prevents bricks from falling inward.
By sliding bricks fore and aft, you can raise and lower the plate to adjust for the exact smoke space. This eliminates a lot of smoke, takes the temperature spikes away from the exhaust and drives the heat forward so the front top will be hotter than the rear top.
This decreases how much you would use damper since this adds a little more resistance through the stove itself.
Does your factory baffle have notches cut in the front corners? This was to allow more heat up at the front corners, create turbulence, and prevent stagnation of moving gases in the corners. It's not necessary, but this design was tested by Fisher research and development to reduce smoke particles which it did very well.
This design was called the Smoke Shelf Baffle. It was made of 5/16 mild steel, the same as the top.
Here is a pic of angle iron welded to 5/16 plate for a Mama Bear; Plate is upside-down to show angle. Notice how brick is held against walls . Put the angle at the front end where it will contact bricks.