Conn 60H "C" with Bb rotary attachment
Date: 1914. Image courtesy Douglas Yeo
As far as I can tell, this instrument either changed name around 1925/1926, or the same model was sold under the Conn name and the Wurlitzer name simultaneously. The brand name "Wurlitzer" was apparently used by Conn for a period in the 1910's and 1920's. This is an actual "C" trombone, with concert-C in first position. The rotor is a Bb (not F) attachment, turning it into a "regular" Bb trombone with concert-Bb in first position. Apparently this was called the "Preacher model", because it allowed trombone players reading in treble clef to play from concert-C parts without transposing, thus being able to play along with the preacher. This remarkably well preserved instrument is owned by Douglas Yeo, bass trombonist of the Boston Symphony Orchestra and music director of The New England Brass Band.
This model 60H was built from 1914 (but possibly earlier) to 1932, and has a #1½ (0.458") bore. That is the same as many modern trumpets.
What Conn said in 1924:
The "C" Model Trombone is especially favored by those who do not desire to transpose from vocal scores. It possesses a rotary valve change slide which throws the instrument into Bb when desired. It is the popular instrument for home and church.
What Wurlitzer said in 1926:
Again Wurlitzer leads. For the first time, we offer the Wurlitzer new Melody Trombone. There has been an insistent call throughout the entire country for a trombone that could be played in C, with the piano and violin without transposing. And heretofore no popular priced instrument has been put out for this purpose. After over a year of experimenting and changing of models, Wurlitzer no offers and fully guarantees this beautiful new instrument. It is built in the well known professional model and embodies the latest improved rotary change from C to Bb, enabling the musician to use it either at home for instant playing with violin or piano or for solo work, or in a second's time, an an orchestral instrument of the highest class. For the first time a high-grade instrument embodying these advantages has been offered for the low price of $29.75. Get in on this early, as we predict a tremendous wave of popularity for this instrument.
Specifications: Medium bore 7¼" inch bell, weight 2¾ pounds. Smooth slide action and perfect intonation in all positions. Nickel-plated mouthpiece and latest split type music lyre included. Improved rotary change to Bb. Triple braces and improved slide-action water keys. Splendidly balanced. An instrument that has proven exceptionally popular with professional and amateur musicians. Built in low pitch only.