I sent an email to Lytle Hoover, and received this reply:
"Actually in our researching our old records we can only estimate only >250 in the US. Additionally Marconi built their own TK-41s from RCA plans in England. NHK in Japan received @10 and built others there also.
Many of the photos that got displayed of local TV mobile units were the RCA unit being loaned or leased to the stations for special events.
-from Lytle Hoover's research of his RCA records to augment Ed Reitan's website's initial study of 'early Color Pioneers.'
http://novia.net/~ereitan/PION_6m.htm#EarlyPioneers
Lytle, former market research administrator for RCA Camden, maintains the RCA TV Equipment Section of Barry Mishkind's "Old Radio" website
http://www.oldradio.com/ "
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Much thanks to him, we are homed in on the probable number built, and it is interesting to note what work-horses they must have been to be kept in service and shared around all that time.