Looking at the 1978 result, 3 shooters were using non German guns, I think the 4th placed shooter must also be commended for overcoming the psychological hurdle of knowing he was up against FWB and Diana which were then the benchmark of technical excellence.
I think that FWB were also perceived as being fully focussed on producing a single competitive gun rather than the more diverse offerings from Diana, which were also more difficult to service.
I presume the Diana's indicated as mod.175, were in fact the mod 75.