For the sake of comparison, here's a damaged 1850 for which a dealer charged me $15 at a show about ten years ago ....


I have yet to attribute this maverick counterstamp. The dealer obviously didn't view the year as a key date, nor do I, but it's admittedly a scarcer year among the Braided type.
If one solely interprets the word "key" as a scarcer, higher-priced coin, the moniker fits though. IMHO, many so-called "key" coins are over-priced in today's market ... for instance, 1921 & 1928
Peace dollars. The "key" then opens the buyer's wallet.
Presently,
ebay lists over 200 1916-D dimes and about 300 1877 Indians. These are generally deemed "key" coins. Low grade examples tend to trade at $400-$500, being the going price of a much higher grade 1850
Half Cent. There are currently but 50 of the 1850
Half Cents up for sale. By comparison, it seems the 1850 qualifies as a "key" coin. Here's a breakdown by the year for this Braided type:
1849 - 50 pieces
1850 - 31 "
1851 - 137 "
1852 - 2 " (Proof Restrikes)
1853 - 159 "
1854 - 83 "
1855 - 104 "
1856 - 41 "
1857 - 66 "
In comparison to other Braided
Half Cents, the 1850 seems to be deserving of being the "key" to the series. The key to fetching a strong price for a coin of this year then depends on finding a buyer who agrees it's a "key" coin. Perception, then, is the key factor.
While I'd sooner own an XF 1850
Half Cent than an AG-G 1916-D dime of comparable value, the latter is more of a bread & butter coin, being more salable in today's market.