Sadly Midland Gun's records fell foul to the attentions of Hitler's bombing campaign in the last war. The wavy lines on the rib, as you say are similar to Parkers, this is another fairly typical feature of Midland Guns, although not exclusively. The number 25302 could be one of their numbers. I have about half a dozen guns by this Maker and all are later than yours and with higher serial numbers as expected. They were into six figure number before the last war.
Quite true, the checkering is not typical British style but again there are exceptions. I have a gun by John Fry of Derby with the same style of checkering.
I have a Demon model in 12 bore that is a later gun and by then it has the cross bolt closure. The 20 bore Demon that I did have (wish I still had) had a top rib extension that was in the shape of an ace of clubs and fitted a slot in the action of the same shape. I can't remember the number but it was a fairly early model with damascus barrels. I would be interested if your catalogue, Roy, shows this feature. Lagopus.....
