Hi angelreader,
What I suspect you are referring to are the cardboard type silhouette models that the US Navy Bureau of Aeronautics produced in 1942-early 1943 time frame. They were die cut into a thick black sheet of cardboard and inserted into individual envelopes along with instructions for their assembly. The instructions sometimes took the form of a separate sheet of paper, others were printed onto the envelope. The first picture below shows an example of the Ju52 in this type. The second photo is of the instruction sheet that accompanies the model.


The Ref No. S-98 indicates that this model was part of the Navy's first series. The S-series was a bit erratic in numbering, nation of origin and issue date. For instance S-10, issue date 6/15/42, was a Mitsubishi 00. S-11 dated 3/5/42 was a German He115.
There was another series produced with CM- reference numbers that from what I understand were a joint services effort. They are labeled As Army Air Forces - Aircraft Identification Section - Wash., D.C. The numbering of these models was more cohesive. CM-1 through CM-17 were all Japanese planes, CM-101 through CM-114 were all German planes, US planes occupied the CM-500 numbers and British planes filled the CM-600s. I believe the CM-series were issued as sets where the S-series were not. Army Air Force identification posters follow this same general convention but are not exact one-for-one matches numerically.
You can see a few more examples on my web site
http://ajsrecog.mysite.com, just select Cardboard IDs from the upper menu.