[Heathkit] SB-401 21 MHz trap

[jack m]

I notice that some SB-401's have the 40-479 21 MHz trap and 180 pf capacitor beside T2, and others don't - but the silkscreen is on all the boards.  Was this a rolling change or elimination on Heath's part?  I cannot find any reference to it, or an adjustment in my manual, so I assume I must have a "late" manual, or did I miss an option to install/not install somewhere?
 
What's up?
 
Jack