Open Style
Low-numbered years represented. Could be simply the style changed at some years-of-service (ala Douglas), but some overlap with the wedge-style pins at at least the 15 year point.

Five year pin.
Ten year pin.

Wedge Style
I believe this style to be contemporary to, or a successor to the Open Style due to an example of both at 15 years (see set below). Also, I've never seen a pin less than 15 years in this style (doesn't mean they don't exist, of course).

Fifteen year pin.
Twenty year pin.
See Set 1 below for a better view of this pin. Twenty five year pin.
Thirty year pin.
Another thirty year pin.

Sets/New Style
I've listed these as sets as they were listed like this on Ebay. As such, they are useful because they might give insight into transitional pin styles.

Set 1 showing three different styles: Wedge, Round Logo, and Square Logo. The Round Logo style can also be found with a red stone. 20, 25, and 30 year pins?
Set 2. A terrific set showing three different styles: Wedge, Open, and Open Enamel. Shows both wedge-style and open-style 15 year pins (intriguing that both are in the same set) as well as a early-style (?) enamelled 5 year pin. Also shown are two vintage promo pins.