US Customs Info Needed On Latvia Registered Cover

This is a registered cover from Riga, Latvia mailed April 2, 1927 to the National Soldiers Home in Hampton, Virginia. The stamps are a margin block of 4 Sc 116 and a block of 4 of Sc 122. The cover measures 9 1/8 inches by 6 1/8 inches. The front has 3 customs hand stamps. The 2 upside down ones read Passed Free of .U.S. Customs Duties, U.S. Customs Bureau Post Office, Washington, D.C. The one is covering up a previous hand stamp which I'm having trouble making out. It looks like 'Supposed...to.....' Can anyone help me with that? The back has been cancelled Washington D.C. Reg. Sec on Apr 16, 1927 which was a Saturday and sealed and cancelled again on Apr 20, 1927 which was a Wednesday when I assume it was passed on to the Soldiers Home where it received a cancel on Apr 21, 1927. The cover also has a 4 digit number on the front crossed out in red which I assume might have been a registry listing here in the US maybe in Washington. The cover has taken a beating but I like it's travels along the way.

I would assume that on arrival, as it was registered, it received a SUPPOSED LIABLE TO US CUSTOMS DUTY handstamp. It would later be assessed and either a PASSED FREE or FEE PAID type of handstamp would be applied.

Thanks a lot scotzm. For the life of me, I couldn't figure out that 'liable to' part.