SB-401 Parts AM filter question?

[FRED HOLNAGEL]

.  For sale excellant panel for the 401 with plastic escutsion plate with "Heathkit" and "SB- 401" emblems sharp and unscratched. All silk screen ing sharp and unworn. $20.00
Panel meter, clear face. $12.50
Power xfmr. $15.00
Set of tube pulls minus the finals and one small tube $10.00        
Other parts, what do you need?

All above plus shipping from 98661

Question:  I have what i think is the AM filter for an SB-303 receiver marked 3.75 khz. (Seems narrow?)  On one side is stamped 2171. Part number is 404-M285P364.  The 2171 has me stymied. (Kit part #?)  Thanks!

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