Franco Belge Top Load Boilers Efficient? Smoke When Loading?

I asked some questions about a Franco Belge 92-125 coal (and wood?) boiler over on the topic:
Franco Belge 92-125 Coal Boiler: I need information. I debated whether to ask these questions there or start a new thread.

Here's the eBay listing of the stove: **Broken Link(s) Removed**

I got some helpful answers on the other thread, but no one who replied actually owned that stove, so I'd like to ask a few more general questions here.

Efficiency question:

The boiler is top loading and top vented. It looks like the path of combustion gases is rather direct from the top of the fire to the top stove pipe vent. I might have expected the gases to be channeled down from the top of the fire to a lower part of the back of the stove (where there is a water jacket for sure--I didn't look carefully to see if there is a water jacket surrounding the fire) to extract as much heat as possible from those gases.

For anyone else running a Franco Belge boiler like that, what do you think? Do you think there might be a damper / vent to redirect the gases that I might have overlooked? Frankly, when I looked at the stove, I didn't think of looking for anything like that--I just noticed that the normal path of gases was up and then back to the vent pipe.

If anyone is running a similar Franco Belge stove, does it work as I describe (gases simply go up, back, and out), and, if so, do you have any idea of the efficiency of your stove?

There is sort of a grate in the path of the gases going to the back, and I don't understand its purpose--it is definitely a grate (as opposed to a flap or damper)--it is movable (I'm not sure where or how it might be adjusted), but there are narrow pieces of metal (cast iron?) with wide spaces between them--if it was moved to full vertical which might be the sort of closed position, there is plenty of open space for gases to get through.

Aside: In the answers on the other thread, it was at least hinted that it might be intended to burn both wood and coal, and that, I'd find the firebox oversized for coal and might need to use firebrick to reduce the size of the firebox when I burn coal.

Smoke question:

When I had considered a Franco Belge boiler some 30 years ago, they seemed to be intended to burn coal, and I was not too worried about opening the top (briefly) to add coal. If I burn wood in it, I'm a little more concerned about smoke and flames escaping when I open the top to load more wood. Is it typically a problem?