That was at a wedding in NJ when he was invited to sit in. I wish I had a photo from his TV interview at Harvey Phillips' Jose Cuervo concert in Grand Central Station. After admiring the patina Al Roker asked "does it play?" Dad responded with a beautiful full-range arpeggio which echoed for several seconds all over the station. On another occasion the actress Shana Alexander asked if it was an "etruscan" tuba.
Yes, the W slide put the tuba into standard pitch. He always used a Martin Eb tuba mouthpiece, gold plated. A bass trombone mpc would fit, but it wants a somewhat bigger cup. The shank of a Kelly 25 could easily be filed down to fit; that would be the right size.