[Johnson] What is a tube shield insert?

[Lee Craner]

Hello list:

As I complete my Pacemaker restoration, I have noted in the manual's parts list, "22.1493 Inserts, tube shield, XS4, XS5."

What is a tube shield insert?  The tube shields themselves are listed separatly (part number 22.1218-1 for the 9 pin variety).  V4 and V5 are the ECC-81/12AT7 balance modulator tubes. 

On a related note, the 9 pin shields for the 12BH7 and 6CL6 seem to be too short (they fit, but with no room for ventalation.  Is this normal, or do I have the wrong shields?

73,
Lee WB6SSW