A few words on engravings
I thought it might be interesting to commit some thoughts to "paper" with respect to the engraving done by the Conn company throughout the years. What follows is not intended as a complete treatise on Conn engraving, but more as to give some suggestions on what to look for. For a more in depth look at Conn engraving, I recommend reading Dr. Margaret Downie Banks' book "Elkhart's Brass Roots".
Approximate dating of an instrument based on the engraving
It is possible to put a (very) rough date on an instrument based on the style of the engraving. From the start of the company through approximately the early 1920's engraving styles are usually floral and/or are depictions of scenes or figures (not including the "Naked Lady" engravings, which I will come to later). From the mid to late 1920's you see floral engravings, but usually no scenes or figures.
Starting approximately in the late 1920's or early 1930's (I am not sure) you start to see art deco style engraving, with mostly geometric patterns. This lasts up until about 1954.
From 1955 all models except the Connstellations have the "three marching men" logo. The more expensive models (Victor) have a leaf-like engraving surrounding the three marching men logo, the less expensive models (Director) don't have the leafy engraving but have what is often referred to as the "shooting stars" engraving. The in-between models (Connquest) have both.
The "Naked Lady" engraving appear on instrument roughly during the 1920's and 1930's.
Some tips on engraving that isn't found on Elkhart Conn instruments: anything with "Director" engraved on the bell. Anything with the model number engraved on the bell (with the exception of the 60B).
Quality based on the engraving
This is a somewhat controversial subject. Let's start with "the big one": the "Naked Lady" engraving. First of all, the "Naked Lady" is not a model, and is no indication of a model. This engraving is most often found on saxophones and low brass. It is in these quarters that you most often hear the term "a Naked Lady model". However, it is incorrect to call the "Naked Lady" a model. As to the quality issue, Dr. Banks in her book "Elkhart's Brass Roots" says: "[N]either the mere inclusion of this design on an instrument nor the extent of the woman's breasts depicted in the engraving has anything whatsoever to do with the quality of the instrument on which the engraving appears." This statement is based on the fact that a convincing argument is sometimes made that only the better instruments that came off the line were chosen for the "Lady" engraving, and the better the instrument the more of the Lady there was, the argument being that Conn wouldn't waste time and effort to put such an extensive engraving on a lower quality instrument. I cannot personally say one way or the other which is true, however Dr. Banks is the foremost expert in this field, so I defer to her knowledge.
During the 1930's through 1941, the "Special" models, most notably the 22B New York Symphony Special, the 38A Victor Special and the very rare 12B Coprion Special, had extensive engraving. These "Special" models appear to have had more care given to construction and quality. It is reasonably safe to say that during this period, the more engraving an instrument had, the better the quality was. Often though these instruments were played "to death" by their (professional) owners and consequently these days it is difficult to find a Special in good original condition. This is especially the case for the 22B Special; I have not yet personally seen a picture of a 22B Special in good original condition, with the original lacquer intact.
Another myth is that the inclusion of the "Three Marching Men" logo on an instrument's bell is a sign of a student model. This is not true. As I stated above, ALL Conn cornets and trumpets (and probably trombones) from 1955 to 1969 had the "Three Marching Men" engraving on the bell, with the exception of the Connstellation models. The difference is in what surrounds the "Three Marching Men" engraving. The more of a leafy pattern there is, the more expensive the instrument. Consequently, the 10A Victor, for example, has an extensive leafy pattern, while the 15A Director has none. The inclusion of "Shooting Stars" does indicate a student model such as the Director, or the Connquest which had both the leaves and the stars.