Did the Illinois State Police use Federal 9BPLE in the model 39, or only in the steel frame guns?
The ISP never issued any steel frames.
I don't recall exactly when the BPLE was issued. In 1980 after a shooting involving the 115 Silvertip we went to the Federal BP round. We carried that for a short time until the BPLE was developed. Then in 1981 we went to the 439. I don't recall if we were issued the BPLE when we got the 439 or just after. Around the same time.
Why were the first 39s issued to the ISP in flap holsters?
There are several advantages to the crossdraw and the flap.
The flap was for retention. At that time there wasn't the thumb break in that style, usually a strap.
When seated in a squad a crossdraw is easier to deploy and it made it easier for weapon retention. With the seatbelt, radio console, etc, the crossdraw got the gun out of all that.
The crossdraw was worn just to the left of the belt buckle in front of the hip. This made for a very quick draw particularly when addressing a violator from the interview stance. The butt of the gun was right at your hand and as soon as you cleared leather you were on target.
Another advantage of the crossdraw it made the gun more easily accessible with either hand. A strong side with belt keepers or velcro belt to draw with the off hand can look like someone dancing with a mouse in their pants.
In the mid 70s one of our Troops was walking up to a car when the driver poked a .25 out the window and shot the Troop in the right shoulder. The round hit a nerve which limited the use of his strong arm. He was able to quickly draw with his off side hand and return fire.