These are the designers initials.
greek
They're the engraver's initials - Dennis R. Williams
It is the initials of Victor David Brenner, the coin's designer.
It is the creator and orignal engraver of the coin.
Your $5.00 gold coin is a 1/10 ounce gold Bullion coin, the letters MB are the initials of Miley Busiek, she designed the reverse of the coin.
The dime
They're the initials of John Sinnock, who designed the coin.
Actually those initials appear on many Canadian denominations and older Australian Coins as well. KG is the initials of the designer (George Kruger Gray) of the reverse (tails) side of the coin.
FG are the initials of Frank Gasparro, he disigned the reverse of the coin. Gilroy Roberts did the obverse.
Presumably you're asking about the tiny letters appearing under the portrait of Franklin Roosevelt on the US dime. They're simply the initials of the coin's designer, John Sinnock.
The initials SWL mean Safe Working Load
Unfortunately the initials killed the collectible value of the coin, it's face value.