12ax7 ecc83 tube NOS Mullard Blackburn factory   I63 B8H3 preamp tube test strong one piece

1  tube 12ax7 ecc83 Mullard NOS , 

Blackburn factory code I63 B8F3

tested on tube tester TV7D/U = 53 & 50 (reference good at 32 & 32) 

grey ribbed plate , O getter 

box 177