The "mousetrap" mag has a loop of spring wire sticking out of the back of the magazine that vaguely resembles the hoop on a mousetrap. The purpose of this wire is to press on the bottom of the butt of the pistol and help the magazine to overcome the friction of the mag safety and drop free. It's not that useful, in my experience. Mec-Gar mags are good quality and readily available.
You got a free C-series Hi-Power - nice score! Those are generally more valuable than the later HP's with 245 serial prefixes - probably $1000 or so, but possibly more in good condition, with accessories, or to the right person. I wish people would just gift me guns sometime.
Buy some mags, shoot it, and enjoy your new gun.