Wikipedia
http://en.wikipedia.org/wiki/Ruger_MK_III says it well:
Changes from the Mark II
Addition of a visible loaded chamber indicator on the left side.
Magazine release moved, from bottom of grip to behind triggerguard.
All Mark III pistols with adjustable sights are drilled and tapped for a Weaver-style scope base which ships with the pistol.
A new magazine disconnect prevents the pistol from being fired with the magazine removed from the pistol.
An internal safety lock has been added. Using a key, the safety can be locked in the "safe" position.
Shorter tapered bolt ears
---Mark III
The first is simply known as Mark III. The pistol's receiver and barrel are made from a single piece of steel, with a layout similar to the Luger P08.
The high-end model is known as the Hunter. This pistol features a stainless steel frame and 6.875-inch fluted bull barrel, fiber-optic front sight, adjustable rear sights, and half-checkered Cocobolo grip. It features multiple safety mechanisms, and includes a visible loaded chamber indicator (The first ever included in a .22 rimfire pistol) as well as internal and external keyed locks provided.
The next pistol in the lineup is named Competition. It too has a stainless steel frame and 6.875-inch bull barrel, and adjustable rear sights; the pistol also has the Cocobolo grip.
Target is the next Mark III. This pistol has a blued finish, a tapered 6.875-inch barrel, and adjustable rear sights.
The Bull barrel pistol comes in stainless steel or blued finish. It has a heavier barrel to help the barrel from overheating and to keep recoil down. It has a 5.5-inch barrel and adjustable rear sights.
Finally, the Standard Mark III has a tapered barrel, black plastic handgrips and a blued finish. It comes with either a 6-inch or 4.75-inch barrel. It has fixed sights.
See
http://en.wikipedia.org/wiki/Ruger_MK_III for model number grid spreadsheet.
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Although they held that specific niche market captive for years, with the higher priced spread provided by High Standard, there have been some recent new players in the market.
Look also at the Beretta Neos, S&W M22... among others.