Frank recently alerted me to this issue as well. The moratorium was
in the early-to-mid-90's, and everything imported after that date is
pretty much subject to this stuff. NOTE that I said "imported" - date
of manufacture doesn't play a role, AFAIK.

I did some research, and here's what I've come up with:

The key to look for is whether the letter is called out as part of the
"Operating Limitations". You'll note that almost all Special
Airworthiness certificates (i.e. Experimentals) in recent years state
that the aircraft is subject to all of the Operating Limitations.

If the SA certificate refers to the letter, or the letter talks about
being part of the Operating Limitations (or part of the SA cert
itself), then its a bundled package that you must consider as a single
document. Operating outside of the conditions in the letter is
considered a violation of the Airworthiness certificate itself.

To change the conditions for legal flight, you must have the FSDO or a
DAR come out, look at the aircraft, and draft up a new Airworthiness
Cert and/or Operating Limitations letter.

I've heard that having the current paperwork and a recent annual (and/
or A&P onsite at the time) helps...

Take care,

--Noel