I'd need to see both sides to see for sure what you have.
FN switched to a cast frame around the same time they switched to the Mk III Hi Powers. It was done to accommodate the addition of the .40 S&W Hi Power as the cast slide was stronger than the forged slide so the .40 needed the cast slide, and the 9mm Hi Powers got it for production commonality purposes.
The purists whine about non forged slides, but the cast slide is stronger.
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Cylinder and Slide will sell you the parts for an extended safety, or they will install it for you. It's uncommon to have a "drop in" safety actually drop into a Hi Power without some minor fitting being needed. However, if you're ok with tools, have some emory cloth and a set of small files, fitting one is pretty straight forward, only takes about 15 minutes, and is pretty hard to screw up as long as you remember to take the metal off the cheap parts, rather than the frame.
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Rear sight options are limited on the older Hi Powers unless you're willing to have the slide milled, and the front sight will require the attention of a gunsmith to properly change it, unlike the Mk III Hi Power where the front sight is dovetailed.