5903 mag release button stiff

[BruceHT]

I recently acquired a 5903 not sure of age. The 3 older mags that came with it release easily. The new mec-gar 15 round mags do not. The button is hard to press. Is that normal for any new mags on this gun or just new mec-gar mags? Will they work in over time or would anyone suggest an extended release button?

Also is there a serial number lookup to see what year this gun built?

Thank you
Bruce