[Elecraft] KPA-500 modifications

[Johnny Siu]

Does anyone have the any idea about the 'Factory upgraded w/ T/R circuit plus 12-meter low pass filter mod'  mentioned in the advert. below?
73
Johnny VR2XMC
      寄件人︰ H. Cary <h3cary at gmail.com>
 收件人︰ elecraft at mailman.qth.net 
 傳送日期︰ 2015年06月10日 (週三) 4:15 AM
 主題︰ [Elecraft] KPA-500 FOR SALE
   
For Sale: Elecraft KPA-500, serial # 0698, solid state linear amplifier, covers 160-6 meters.
Factory upgraded w/ T/R circuit plus 12-meter low pass filter mod on June 1, 2015.
Amplifier "meets or exceeds all factory specifications."
$1750 shipped CONUS.  Please contact direct: h3cary at gmail.com
Cary, K4TM