Deaf, my favorite manual safety add-on for those who want one on a Glock is the one developed by Joe Comminolli (hope I spelled that right). It's a thumb safety mounted on the frame, very ergonomic, works like a 1911's in that up is "safe" and down is "fire." Can be quickly and economically installed by Rick Devoid at Tarnhelm,
www.tarnhelm.com. Only down side is that it's right hand only.
best,
Mas