I have a Liberty nickel graded by ANACS as MS 62 with variety PM-19.3 Can anyone tell me what the 19.3 means?

From the ANACS FAQ page:

What are varieties?

The advent of modern minting technology has enabled the production of huge volumes of virtually identical coins. They are struck on high speed presses using tools called dies. Each die is a negative image of the coin it is intended to produce. The dies are in turn produced by being impressed by a tool called a hub which contains the features of one side of the coin it is designed to manufacture. The hubbing process is used to make dies of a consistent quality. Within a given year's production, slight but noticeable differences may occur in the die making process. Coins stuck from these dies are known as varieties.

What are variety numbers?

Coin collecting is an information driven hobby. Collectors of varieties of a particular series often rely on a standard reference book that lists all the known varieties or the most significant varieties of their chosen coin series. Listings in these books usually are numbered. When referring to such a listing, it is the standard practice to abbreviate the author or authors name(s) followed by the number they have assigned to a specific variety. For example, collectors of Capped Bust Half Dollars use the standard reference Early Half Dollar Die Varieties by Al Overton. A specific coin such as the 1823 "Ugly 3" would be listed as O-110a, the number Overton assigned to this particular variety. Works of this nature exist for many US coin series.

Also see

http://www.libertynickels.org/variety2.php?&page=variety&lnid=1004