Why do some Glocks have a second pin hole above the trigger pin?

[Linea_de_Fuego]

Can some one explain about the function of the second frame hole and pin above the trigger pin in some Glock models or generations. I have seen G19 models with and without it and don't understand if one is preferred over the other.
http://content.answers.com/main/content/wp/en/thumb/f/fd/325px-G19_blk_grip.jpg
http://img.timeinc.net/time/daily/2007/0704/wviewpoint_0430.jpg

[Robb Jensen]

All new 9mm frames have the same locking blocks as .40 and .357 frames thus they use the same top locking block pin. The 9mm didn't need it but Glock decided to streamline the parts inventory and having just 9mm locking blocks was a waste.

[Business_Casual]

The heavier recoil of the .40 and such rounds required a more secure method of securing the recoil lug in the polymer frame. Later on, when frame production began in the US there was no reason to continue to make a two-hole and a three-hole frame so Glock just makes all, including 9mm, three-hole frames.

M_P

[Linea_de_Fuego]

Thanks for the answer. I used to have a G 19, 2nd generation like the one pictured but sold it. BIG mistake, one of those things if I only knew then what I know now.