What this coin looks like (obverse, reverse, mint mark location, special features, etc.): There were two major varieties of the 1907 Arabic Numerals (Low Relief) $20 Saint Gaudens Gold Double Eagle. One variety is unique and features large lettering on the edge of the coin while the other is the common variety with small lettering on the edge.
Due to technical difficulties and requirement of multiple strikes for each coins with Ultra High Relief and High Relief - the US Mint decided to change the system to be more practical for minting coins. The new system required only 1 strike per coin - which resulted in a Low Relief field. The design is also changed as well - particularly with the date. The MCMVII Roman Numerals were used in all High Relief coins - but this was later changed to the common "1907" Arabic Numerals afterwards. So any "Low Relief" and "Arabic Numeral" varieties are one and the same.
Depicted below is an example comparison image of the edge lettering of the coins - Small Edge Lettering vs Large Edge Lettering. Only one unique coin is known to exist with the Large Edge Lettering. It turns out that the collar used on this unique coin was also used on the High Relief coins. It is also very possible that more of these coins exist but are not yet known. Source of example images below is courtesy of David J. Camire:
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD