Well just looked at my old FAC, and guess what, it does have an expiry date on it, 5 years after the issue date....so again I don't see the difference...between it and the current PAL...
unless your FAC predates mine and it was different.....way back then.....
Main difference is if the pal expires and you have guns stored in your possession you are breaking the law. If the fac expired you just couldn't buy more guns/ammo, the stuff you had was fine.