With thanks to Frank for the pics of this (refinished) gun. He says this:
"According to the former owner it is a model 4 from about 1900-1913,thats all he knew. Have been through all my books and other sources ,but can't find any more info about this particular model. Its overall lenght is 96 cm and the barrel is round to actagonal and it has a cast iron buttplate,so that rules out the models 3 and 6 from 1913..."
On the condition of this example he says:
"The only problem was the polishing to get all the pitting out prior to the re-finish.... Almost all of the marking on top of the barrel have disapeared.
At the right angle in the sunlight you can (barely) see the remainder of "DIANA-LUFT-GEWEHR" ,I have tried to take a pic, but to no avail."