S&W made just a ridiculous load of these magazines, they might contend for the single most made pistol magazine in S&W history. There are sooooo many out there that if money is any issue, please don't overpay to get a few. These are easy to find.
The Mec-Gar is a perfectly great magazine, it just looks a little different.
Good luck! I have a 659 also, it's a big 'ole brick of an old school pistol. Yes indeed, it is a 2nd Gen. It's the all-steel evolution of the original Model 59. The 459 is alloy frame, the 659 is all steel.
Hint: fantastic aftermarket grip for a 659 is the now out of production Uncle Mike's grips. These will fit the 59, 459, 559, 659. They are fairly thin but grip well, far better than the original hard plastic grips on the 659.