United States : Auxilliary Markings.

Czechoslovakia (LIB???) to NEWARK or NEW YORK?
21st August 1984
Diamond strike in purple /magenta

U.S. CUSTOMS / PASSED FREE / 398 / NEWARK or NEW YORK
Circular strike unreadable.

Seems unlisted at Auxilliary Markings Club.
http://www.postal-markings.org/listing.html

U.S. CUSTOMS
PASSED FREE
1998
NEW YORK

Being a customs stamp, not a postal service stamp, might be why it is not listed as an auxiliary marking?

A valid question.
I was going on the links' customs listings.
Other than that, I an ignorant of things US, in this discipline.

PS: I think the Czech Pmk is LIBOCHEC.
I also passed the scan on to the editor of the stamp Link in case of any interest.

PPS : With respect, your 1998 is incorrect, it is 398 the custom's ID number.
Also, I don't think NEW YORK fits in the space, it is probably NEWARK, New Jersey.

FYI
Sundry US Items for sale locally in Australia. December 2019.