With all due respect to the hard work and study of the subject done by Stampman22, I'd like to add to his presentation of the stamps of the 1934 National Parks issues and their 1935 reprints.
In his narrative, Stampman22 mentioned more than once that finding postally-used examples of many of the varieties was problematic. Over the past 20+ years, I've been lucky enough to locate used examples of all the different varieties of the reprints of 1935.
As stated by Stampman22, one of the most elusive examples are first day covers of Scott 766 to 770. One can find many examples of singles, pairs, gutters, etc., but it seems very few collectors of the period considered FDCs of the entire sheets. Here are some examples.
The first is a full sheet of 25 stamps, Scott 766. In this case the collector used an oversized envelope without an accompanying cachet. The distinction between this sheet and its look-a-like Scott 730 is the very wide left margin of the sheet.

Here is Scott 767 sent to the same collector. Note again, the wide left margin that differentiates it from Scott 731.

Next is Scott 768, 3˘ Byrd Issue (Reprint), sheet of 6, entire, Planty 768-2, Ioor cachet, tied by a "Washington, D.C., March 15, 1935" 4-bar (long) handstamp cancel to Hartsdale, N.Y. on a 3#8541; in. x 6˝ in. cream envelope. Note that the sheet has small margins similar to Scott 735 but the 3/15/35 cancellation and the use of glue to secure the sheet attest to it being Scott 768.

Next is Scott 769, 1˘ Trans-Mississippi Philatelic Exposition (Reprint), sheet of 6 entire, Planty 769-34b, Top Notch cachet, blue, tied by a "Washington, D.C., March 15, 1935" 4-bar (short) handstamp cancel to Washington, D.C. on a 5˝ in. x 8 in. cream envelope. Again, this sheet has small margins similar to Scott 751 but the 3/15/35 cancellation and the use of glue to secure the sheet attest to it being Scott 769.

The last of the set is Scott 770, 3˘ National Parks Mt. Rainier (Reprint), sheet of 6, Planty 770-12, Fairway cachet, tied by a "Washington, D.C., March 15, 1935" 4-bar (short) handstamp cancel to Riverhead, N.Y. on a 3 15/16 in. x 7˝ in. cream envelope. Notice that the sheet has a large left margin clearly indicating it is Scott 770.
