Likely 1962 or possibly 1972.
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Xf1 coding indicates Mullards made in the Blackburn Works, in the U.K., from the late '50s to the early'60s.
Xf2 indicates either the Blackburn Works in the U.K., or Eindhoven, Holland tube factory (after Philips Electronics bought Mullard in the '60s)
The differences in the 2 facilitiess tubes are in the bases -color and size, and the getters. Both facilities made these tubes for other companies (for re branding) as well.
In the case of the 1st letter, ie: 'B' for xf2 that's the factory code. Xf1=Blackburn, Xf2=Blackburn -or-Eindhoven, Holland. The 9 would be 1969 (in the xf2 case). The 'H' is the date code month -August. (A= Jan, B = Feb,... H = Aug), and the '2' is another date code, the week. in this case the 2nd week of August.
In the 'Xf1' case the 'B' is Blackburn, the '1' is 1961, the 'E' is May, and '2' is the 2nd week of May.
In the case of your tube, most likely manufactured in Sittard, NL between 1957-1962 (Codes): Xf5 L7X - LXX, Identified by: - DD getter- brown Bakelite base.
These are quite rare and supposed to sound great. I don't have any of these to confirm by listening test.