I found the rifle for sale in Norway now

http://www.beverensport.no/products/gustav-genschow-co-modell-98

in the 2004 Widforrs/Stockholm catalog where it was sold for 4200 swedisch krona - now 3531 norwegian krone. the rifle was also in 10,75x68 so the rechambering may be done before the 1990s when RWS stop making this round. the rifle was thean sold with a weaver 2,5 scope

december 2004

Tyskt, repeter, fabrikat: G.G. & CO. AG.,
kal. 10,75x68, tillvnr. 986, piplängd 63 cm,
fast sikte med ett fällbart siktblad,
halvoktagonal pipa med hel spång,
plundringsbart magasin, kolv med kindstöd
och pistolgrepp, bakelitbakkappa, stockbult,
rembyglar, Weaver 2,5x i "see-tru montage",
kolven spräckt och lagad med en extra
stockbult, i övrigt GOTT SKICK.Utropspris:
2000-3000
Såld för 4200 SEK
KULGEVÄR

the desciprion mention an additional rear sight leaf and when I look again at the pics it seems so.