Thanks for uploading the pictures!
That´s a neat stove, my first guess would be 1930´s - early 1950´s.
Concerning the fuel... the grates are that of a "Allesbrenner/Dauerbrenner" - translates roughly to something like "burns everything/continuous burner". It is built to burn any kind of fossil fuel: Lignite briquettes (shape & size of regular bricks) would work fairly well, anthracite/bituminous briquettes (size of an egg) or "propper" - I should say "not processed"- would also be fine. The shaker mechanism of my stove is pretty rubbish but it is only a mid-range product, built right before WWI. It is designed to burn mainly lignite, so it´s primitive grate is ok for that purpose... yours however seems a bit more effective!
Besides the grates there are 2 more indicators that might tell us what your stove was meant to burn:
1: How is the exhaust handled in your stove? Do the hot gasses exit in the rear, right under the cook lids? Or is there a flue in the back, directing the exhaust downward and up again? If so it´s an indicator that you have one of the better stoves for lignite briquettes; that fuel burns with long flames like wood and needs a long flame path to transfer its energy to the stove.
2: Mid-range stoves for bituminous coal/ hard coal were built in a more solid way (thick castings, no steel), but would vent the gasses right into the chimney without a backpipe to extract more heat from the exhaust. Only the better, more expensive stoves came with an elaborated system of flues to extract as much heat out of the gasses as possible.
But I digress...
How much coal will it hold when filled to capacity? I´d say your stove should keep a fire over night if you can fit in more than 10lbs and IF (and that´s the important bit) the doors shut off unwanted air... maybe rope gasket is the simple key to your problem. M.Pierre nortcan
is the expert for any questions concerning rope gasket; other users have successfully worked with thin layers of high temperature silicone around the rim of the ashpit door to stop unwanted air from entering the fire pot.
Maybe somebody else with knowledge on that will chime in as well....