Coaly, What is this?

"Fisher Fireplace Insert". No bear name given. The flue connector was added on for direct connection to a liner. That connector piece is called a "Boot". The outer shell in the back was cut for the blower shell. They were designed to be installed into a fireplace and vent up the existing flue. This practice is no longer recommended, since many oversize flues lower the flue gas temperature reducing draft and create excessive creosote. They also needed to be removed to clean debris that falls behind the insert when chimney cleaning. A boot should always be installed with liner. The blower added on the back prevents inserting into hearth, so it was probably used as a free standing unit. There is an opening under the front to blow air under, up the back, around the flue exit pipe and out the front at top. They will naturally convect and circulate in that direction, but the addition of one or two blowers helps greatly.
Original shown below;

There were many blower designs, all made to mount on the front. The back half is double wall for convection heated air. The front that sticks out of the hearth is single wall for radiant and cook top.
Manual for the full size Insert here;
https://www.hearth.com/images/uploads/fishinsertmanual.pdf

There was also a smaller Cub Insert, Honey Bear, Tech IV, and Polar Bear for zero Clearance fireplaces.

The Insert was UL tested and Listed to # MH11652. (notice the tag on the right front corner of the one pictured above)
Look for a stove with a tag. It will have the arched top doors. They were all tested, just not to the nationally recognized UL standard that didn't exist when yours was built.