Mullard 12ax7 dating
They switched over to the 15mm short plates (which are excellent as well) in the late spring or so of 1959, there were up to a few months of overlap.
Through late '62 the Dutch short plates had larger halo getter rings than post-'63 versions.
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For the first part of the alpha code, the K designates a valve manufactured to specification K1001 or K1006; similarly a J designates a valve manufactured to a JAN or MIL spec.
The second letter of the first part of the alpha code describes the qualification approval the valve type has, a B denotes qualification by a UK authority, a U denotes US qualification, a D denotes Australian qualification and an X denotes Director of Electronics Research and Development (DERAD) approval.
The short plates still have the delta factory code, but typically carry I65 type codes, never m C. Vox: 65 AC30 SRT, 64 AC50 Mk II, 64 AC4, 65 tube Cambridge, 66 tube Cambridge, 66 tube Berkeley, 65 tube Pathfinder, Pathfinder 10, V807 echo reverb; 61 Klemt Echolette B40 combo, 64 Fender BF Deluxe Reverb, 67 Fender BF Super Reverb, 68 Fender SF Bandmaster Reverb, 74 SF Vibrolux Reverb, 64 Silvertone 1484, Laub Unique 26R, GK 206ML/E, Leslie 145, Leslie 860...
Klyniss- The factory/date code on Philips (the second line, usually) including Dutch-made from mid-'50s thru '70s with a few exceptions is just that simple as you said above.