Very surprised that this didn't get any bids, as it is a common 21" tube for RCA sets:

http://cgi.ebay.com/OPTICAL-21AMP4-PICTURE-TUBE-NOS_W0QQitemZ220330667629QQihZ012QQcategoryZ67816Q QssPageNameZWDVWQQrdZ1QQcmdZViewItem

Maybe someone can send a question to see if it is still available.