how do you document dates of ratings?

so, as usual before retiring to bed, I am going through
the scriptures, i.e., 14 CFR, and tonight I was perusing
chapter 61 -- which I am sure is on everyone's bed stand --
and came across a part that I haven't fully memorized yet :-)

This is part 61.31(j)(2). Part 61.31 covers those endorsements
one has to have on one's logbook in order to fly tailwheels,
high-performance, complex, etc. aircraft, and subpart (j)
concerns those endorsements required of glider pilots in order
to perform aerotow, ground-tow or self-launch procedures.

Now, like similar clauses, this one has a 'grand father'
exception, which is 61.31(j)(2), but with a twist: unlike
similar 'grand father' clauses which usually refer to time
logged prior to a given date, this says that the
aforementioned endorsements are not required of pilots who
have had a glider *rating* obtained prior to August 4th, 1997.

How does one documents / proves that one has had a rating
prior to a given date? by looking at the date on your
certificate or on the FAA online database? well, no,
that won't work because these dates happen to be those of
your latest rating or latest change of address...

For some reasons I keep old papers and found a certificate
dated 1996 with my private glider rating on it, which
I photocopied right away and stuck in my logbook (would it
be sufficient?) so I should be in the clear on this one.

But think about this, even if you are not a glider pilot:
a similar clause might eventually pop up in the future. Do
you keep all your old certificates? those that we replace
when we get new rating or a new address?

Another odd thing about this 61.31(j)(2) clause is that it
says 'rating' without specifying 'issued under this part' as
is the case elsewhere (e.g., 61.123), so I reckon that any
old odd pilot certificate or license from any ICAO country
might have done just as well (but then I am just speculating
here, if anyone knows better I'd be grateful for the input).

In summary, how does one documents that one has received
a given rating at or prior to a given date?

--Sylvain