Since no one has chimed in on this, I will tell you what little I know about this. I've collected Monon photos for about 15 years and have never come across photos of the cars you are asking about. In fact the only photos of Monon reefers I have ever seen were in company ice service from the '40s & '50s. Until now I had not heard of the decal set you mention. There was an Athearn (Bev Bel?) car with lettering that may have been inspired by your set. It has the same road number and even a curved builder's stencil. This model also has a "MONON ROUTE" losenge herald that was used on freight cars until about 1910.
The Monon had two groups of their own reefers, but both pre-date the decal date you have, were owned by the railroad and would have been lettered C.I.L.
Having said that, I have found Champ's sets to to be accurate, if not always exactly to scale. They even had a set to letter the Monon's 6(!) company service tank cars. So there may have been a car that the set represents.