This ought to take off and run...
Thinking about it... with my statistical handicap of collecting/focussing on MOUNTED groups... I'd have to say I've seen a
few...
but of course, with LOOSE ribbons, who's to say WHEN?
The only Imperial ones I can recall were what I'd consider 1930s or later replacement ribbons, Hindenburg Cross vintage, artificial material, not silk.
A few Third Reich ribbons I never had any doubts about, but definitely still only a minority compared with "straight cuts."
Musing on 35 years of random acquisitions, my gut feeling is 1930s on, probably.
Maybe.
Possibly.
Bets being shrubberied...