Look for welded plates rather than hole punched and tabbed plate. If there are holes it is inferior to the welded version. Date codes are valuable in determining where and when the tube was made. 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 facilities's tubes are in the bases -color and size, and the getters. Both facilities made these tubes for other companies (for re branding) as well. This re branding is mentioned in the post by Ecclectique.
In the case of the 1st letter, I.E.: '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. Etc. for the other tubes.
Hope that helps you identify what your searching for.
