The "spacer block" in the GI holster was designed to keep the mag release from being inadvertently pressed, thereby ejecting the magazine. A new El Paso Saddlery 1911 holster I have has the same "spacer".
If you think that the GI flap holster is a dog, you oughta see the canvas flap holsters we were issued in the Canuck Army for our Inglis-made Browning Hi-Powers! The WWII version was a 'one size fits all' that also accepts revolvers (Webley MkVI and Colt NS) and the later model was damn near bomb proof, making it impossible to get the gun out in a hurry!