I agree, it is very probably a Systron Donner 6053, 3 GHz, nixie. Specs and (bad) pictures scanned from the manual on my server in http://www.spinelectronics.eu/ftp/SD6053.pdf I bet it comes from some remains of John's Radio stock - I remember seeing many of them there, 15 years ago. 73 - Marco IK1ODO