Date: 1950. Owner unknown
This trombone has a medium bore (0.522"), which is just shy of a #3¾. Tuning slide is in the bell, which is the normal place these days.
There is some confusion (at least I am confused) as to when the 78H was produced. The model number list by R. Jones lists the 78H as being produced from either 1921 or 1922 to 1941, then reinstated for several months during 1953, and again from 1959 to at least 1965. None of the Conn catalogs up to and including 1930 mention the 78H, making the first production year no earlier than 1932. The above pictured instrument and the 1950 Conn catalog prove beyond doubt that the 78H was in production in 1950. My guess is that the 78H was first produced sometime during the 1930's and remained in production until the early or mid 1950's (since I don't have catalogs for 1951-1954, I can't determine the exact date). It was definitely not being produced from 1955-1958. It was then reinstated from 1959 into the 1970's.
During the early years of the 78H's production (probably until 1941) there was a choice of bell sizes: 7½", 8" or 8½". The standard bell though was 8", and probably from 1945 the bell was always 8". Also note that during that second production period there was a 79H which is identical to the 78H except that it had an F rotary attachment.
What Conn said in 1938:
The 78H Symphony model is made in a medium-large bore, which fits it for a wide variety of uses. The recent tendency to larger bores has made this model much in demand in popular dance and radio bands, theatre orchestras as well as in concert band and symphony. Standard bell 8 inches. All three models [4H, 6H, 78H] Bb, bell forward in conventional position. Tuning in bell; slide lock and bell lock.
What Conn said in 1966:
Popular with professional "studio" men, ideal for symphonic and jazz. Produce[s] a rich, dark tone, throughout somewhat lighter than the 8H and 88H. Brass bell. The 79H has F rotor with E pull. Bore size .522". Bell 8". Length 45 1/2".