This magazine follower is designed for competition which means it is reliable but it does not lock the slide open.  If you look at the photos, you'll notice the "shelf" that normally pushes the slide lock open is angled down such that it will not engage it.

These followers fit all Para pistols and fits the Arrendondo magazine spring.  I'd recommend you use the Arredondo spring so the follower is properly held in place.

This listing is for one follower.  The photo that shows the spring attached is so you can see the fitment.  The spring is not included.