[Heathkit] SB-401 Modification Identification - 21 MHz Trap

[jack m]

I am restoring a recently acquired SB-401 which contains several homebrew modifications.  I've identified and documented all of them, but cannot determine the reason for one of them.  My plan is to remove all of them and restore the unit to original configuration. The mod in question is a series LC circuit with a 20 pf capacitor and a large adjustable inductor (1/2" dia x 2 inch form with about 25 turns/1 winding and ferrite tuning core) installed across the 21 MHz trap to ground in the vicinity of V4.  The work was well done and with thought to layout, mounting and neatness. Is anyone aware of the reason for this particular mod, or if this was a recommended addition or change way back when and what operational enhancement was anticipated? Thanks  Jack  W3RU