I have been restoring a fair few black and white photos on fibrous cardboard recently. This type of photo seems to have had the light-sensitive emulsion painted onto the board and then exposed to light. I would suggest that such large sizes of paper could not be made so the photographer simply grabbed a stable matt cardboard base and painted on the chemicals. The resulting image is a very soft focused photo without any hard defined edges.
With this photo board, it has a matt finish and absorbs moisture very well. If you have any very large old photos, perhaps stored in the loft, still in a frame in a plastic bag, please dig them out and put them somewhere dry and warm before they suck up the moisture in the cool damp air, circulating around your loft. As they do this they swell a little and often grow mould of varying types. The fine black soot-like mould and dry white, spidery feather-like mould, possibly mildew. Neither of these does your photos any good, its best to dry them out slowly and then dust them off very lightly with a soft artist’s paintbrush. Once the worst is off, use a little photographer’s canned air to blow away the spores, but do this outside otherwise they will just settle in the house and not too close to the photo either.
Once it’s all clean get your photo restoration done. The process of degradation is already happening and there is not a lot you can do to stop it! Click this link to see a post on photo restoration of board based photos