The doc says about the stock 40m LPF ...
"The insertion loss across the band is low and the harmonics are suppressed but not as much as the FCC regulations (Part
97.307 says “(d) For transmitters installed after January 1, 2003, the mean power of any spurious emission from a
station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must be at least 43 dB
below the mean power of the fundamental emission.”
... but aren't the harmonics from the Si5351a already >10dB down on the fundamental, so the over all spurious emissions after the 40dB suppression of the LPF are already much greater than the 43dB required by your USA regulations?
The 'optlowpass' filter sure is impressive!
73, Bob ZL1RS