Ronin's probably right. Crystals from different vendors often have different side wall thicknesses. So if your rep was designed for a thick-walled crystal & you replace the crystall with a thinner-walled crystal, the bezel retaining ring will be loose. Or, at least, will no longer snugly fit & seal around the crystal. There are also a variety of crystal retaining rings for rep cases. Their differing angles, heights & overhangs often make them non-interchangable. So it is usually easier, when replacing a crystal, to get another crystal to fit your retaining ring vs vice-versa.