RGSProThe silenced version doesn't use o-rings. It uses a thin rubber ring around the slider that basically looks like the silencing rings sold on eBay.
r3alityTechnically the silencing rings are a very soft microcellular urethane foam supported with clear polyester. They are definitely far from the dental bands or rubber o-rings typical of silencing MX keys. It is a very specialty material that is a pain to source in small quantities and requires specialty equipment to cleanly cut.
rextasyHave you heard any reports of the material wearing down over time? Just wondering if, or how long it takes for their silencing properties to degrade.
kineticThe ideal temperature conditions for the rings are 12-27C (60-80F), humidity between 40-50%, and no direct exposure to sunlight. Under these conditions (likely the conditions of your keyboard) the rings have an acknowledged shelf-life of five years; however, trials have shown the urethanes generally demonstrate little or no degradation in their original properties well past five years.