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 mark "DAJ" on a 2-pound gold coin typically indicates the initials of the designer or engraver responsible for the coin's design. In this case, "DAJ" stands for artist David A. Johnson, who created the coin’s design. This marking is part of the tradition of including the artist's initials on coins to credit their work.
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.
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.
FG are the initials of Frank Gasparro, he disigned the reverse of the coin. Gilroy Roberts did the obverse.
The initials SWL mean Safe Working Load