I've attached a photo of an Iron Cross in my collection which is complete withit's original non-combatant ribbon -as you can see, the colours are reversed. By the way, anyone thinking of buying a non-combatant Iron Cross should take care -because only about 13,000 were awarded they command about double to three times the price of a standard Iron Cross. Obviously the only difference is the ribbon, so make sure that the non-combatant ribbon is an original and not a modern replacement -check the weave and texture, and subject it to the black-light test as well. if it glows, then steer clear.
The best advice I can give as regards your court mounted pair is this:
An Iron Cross with a non-combatant ribbon should ALWAYS be mounted with the non-combatant version of the 1914-18 Veteran's Cross
That said, the reverse isn't true, and it's perfectly feasible for an Iron Cross with a conbatant ribbon to be worn with a non-combatant 1914-18 Veteran's Cross. So on that basis your group seems perfectly legit.
I think that many people who on paper maybe should have received the non-combatant's Iron Cross actually received the standard version -certainly I've seen photos of medical personnel and (I think) Chaplains wearing the normal ribbon, thus validating the group you have. many awards of the non-combatant Iron Cross seem to have been made more for services to the war effort -this is somewhat recognised by the fact that in WWII the non-combatant Iron Cross was not introduced, and was basically replaced by the War Merit Cross/War Merit Medal series of awards.
The "600" observed on your cross is a bit of a puzzle. logically it could be the silver content of the cross frame and ribbon loop, as that's what numbers stamped on Iron Crosses usually refer to. Stephen Previtera's "The Iron Time", plus another privately put together list I have, list all the observed numbers on Iron Crosses as the following:
BD800 -Unknown Maker PS800 -Unknown Maker
CD800 -Unknown Maker S-W 950 -Sy-Wagner (Berlin)
D.R.G.M 653146 -Paul Maybaur (Berlin) Wilm 800 -H.R. Wilm (Berlin)
K800 -Unknown Maker
K5 -Unknown Maker
KO800 -Koniglisches Munzamt Orden (Berlin)
LV26 -Unknown Maker
In addition, the following silver content marks have been observed:
The word "SILBER"
A quarter moon with a crown and a silver mark (usually .800)
The following content marks on their own: .500, .750, .800, .900, .925, .935, .938, .950
So either your cross has maybe a .800 rather than .600, or it's one of the many crosses which have unexplained markings. There were so many makers of the Iron Cross in WWI that many crosses carry stampings or maker's marks that with the passage of time are simply "unknowns".
I hope this has been of some interest,