Today I was making one of my occasional forays to the Library of
Congress in Washington, the world's largest library.  Their holdings in
international auxiliary languages may not be the largest (I speculate
that those are in Europe), but they do have quite a bit of material. 
Among other things while I was there I looked at most of their holdings
in the original Interlingua, Latino sine Flexione.

Sadly, some of the materials are deteriorating physically, just as is
so with many of their materials on Ro.  Even items which are only
eighty years old are starting to crumble, as they were printed on
(apparently) high-acid paper.  There are a number of booklets (I
estimate 12.5 cm. X 22 cm. -- I didn't have a ruler with me) which came
out in the 1910s and 1920s, mostly under the name of Giuseppe Peano
himself.  They are stored in envelopes and are literally falling apart
to the touch.  There is a lot of repetition in the text, so one can
collate most of the texts for the content.  There are also a number of
bound volumes of "Discussiones," "Circulare," and "Schola et Vita"
(especially the latter, from 1923 (IIRC) to 1937).  Fortunately the
issues in the bound volumes have not deteriorated so badly, although
they are at risk.

There was far too much material to go through in a few hours.  However,
I was able to draw some conclusions.  The usages of various early
Interlinguaists were not totally uniform.  A few people even tossed in
an extra verb inflection or so, contrary to Peano's basic idea.  But
from what I did read in several hours, I have come to the conclusion
that Peano himself was the exemplar of "pure" Interlingua/LsF and the
one to be emulated (just as many Esperantists look to Zamenhof as an
exemplar to be emulated).  In that regard, the double volume of 1931,
the "Key To and Primer of Interlingua" and "Primo Libro de
Interlingua," published in London (I own a copy of the combined volume)
adheres closely to Peano's standard.

Whether materials of this sort can be salvaged, at least in the USA, I
would say is doubtful, as I presume that there is limited money and low
priority for conIALs.  This is a shame.  Although I have no illusions
about it making any kind of comeback, original Interlingua/LsF is my
personal favorite.

Paul Bartlett