I believe that the flush fitting 17rd magazines are for the "new" 2022 FN Browning Hi-Power Series, and they don't fit in the original P35 series HP's like you currently own. Meg-Gar only makes the 10rd, 13rd & 15rd in flush fitting magazines. If you get lucky, you may run into some discontinued Meg-Gar 20rd magazines that hang out the bottom about 2". They are more reliable than the 15rd flat hollow base follower. I only use the 10rd, 13rd & 20rd when I break out a BHP.
Years ago there were South African (KRD) 17rd magazines for the Hi-Power. A bear to load 17 rounds in, unreliable, and known to split down the back spline when left fully loaded over-night. Avoid if you ever see them for sale. And they were not flush fit if my memory serves me correctly.