The bottom rib is marked 5624 10 158 11.8 Haven't figured that one out yet.
That would be the preliminary datastring on the 1st pass - #5624 - 10(not sure) - 15*8mm and 11*8mm 1st past diameters.
If I could see it I could interpret it.
Viennese style forend lever:
>>The grip catch was patented by Charles Harvey, an employee with Wilkinson & Son of 27 Pall Mall, London in patent no.1793 of 7 July 1866<<https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=267208&page=all
Almost all of Hannover firearms merchant H. Scherping's wares wore that style forend fastner.