The PT15 output pentode is a development of the Marconi Osram Valve company type PT5. The principal modification was to raise the filament voltage from 4 to 6.3V and then fit a ceramic base to 'tropicalise' the device such that absorbtion of water by a lesser base material type would not cause parasitic oscillation and arcing problems in usage. When you consider this usage was primarily in airborne bombers like the Lancaster and Mosquito you can appreciate why this costly measure was undertaken. The final modification of course was to improve the support structure and rigidity of the electrode cage by beefing up componentry yet wherever possible reducing mass.
Ally this build quality to simplicity and you can appreciate why these valves were so successful in push-pull configuration as the PA stage for the Lancaster bomber transmitter, the T1154 and why today these are prized by audiophiles as their appearance as well as the sound they produce is superb.
The PT 15 or VT104 in Air Ministry parlance was so essential to the War Effort that production was given over to a shadow factory in an area of Oldham called Shaw near Manchester and virtually every PT15 ever manufactured was made there. Indeed, Cape Mills still stands today and post war, became a Ferranti faciltiy before being wound down and today being given over to small business units. This mill is only some 25 miles from Mullard Magic HQ so it seems the devices we have on offer may not have travelled far in their 70 year lifetime!!