The 503 was a rather unique low freq tube scope with 2 identical input amps in addition to the time base. I got one in 1972 and used it occasionally until 2008. Unfortunately, I couldn't give it away when I moved. For audio work it was great. BW was 455khz I think at 1Mohm. Maybe 10Mohm. Was made in late 50's to early 60's. I never had to touch anything inside in my 36 years of ownership, it just worked.
