Here's a blurb about it:
The Conn 66H is an interesting trombone. The 66H was produced from 1920 and discontinued in 1925. The specifications are .547” bore medium bass with rotary valve to F, tuning in the bell. My instrument was built in 1938-1940 and is marked 66H Spec on both the bell and slide section. The entire instrument is built from either red brass or gold brass and you will see the variations in material in the photo. The instrument is nearly identical to the Conn 88H in every way. The tuning slide, hand slide and bell taper is the same as my later examples of 88Hs (and are even interchangeable), leading me to believe that this instrument is perhaps one of the earliest examples of what would become the modern 88H. As the 88H was not produced officially until 1954, perhaps the 66H was the closest thing to these specifications in the Conn archives and this instrument was stamped as such. I also have a 14H in my collection, which is the same as a 66H but with TIS.