Kuduae, thanks for the additional information, all very interesting & welcome. Lars, when I get a chance I will have a closer look at the buttplate. It is very smooth & I don't see any of the 'grain' that is quite obvious in the horn underlever, so I expect it to be a pressed buttplate rather than carved, possibly the Ebonite that Kuduae mentioned. I have seen horn buttplates on other guns that have started to flake, almost looks like they are delaminating, mine shows no such symptoms.
I really liked the Gruendig invoice. I see that it was dated 1926, would the Carl Gruendig who made my rifle have been around then, or was it just the business name that was still in use? I see the invoice shows a Max & Hugo Gruendig below the business name, I assume they are Carl's sons & would have taken over the business. Does anyone know if the business is still operating, or did they go out of business or possibly bought out by a larger gunmaker?