Thank you for the kind words (blush)
Believe it or not, I have found that unless someone abuses their gun, or something really drastic happens, the internal parts of a Six series rarely just go bad. Now that said, any of the parts that touch each other/get rubbed/or get exercised a lot (springs) will go bad over a period of time but if the owner takes just a modicum of time to maintain their gun, I believe they will last with no or very minor repairs for many, many years.
If I were to keep anything available for that freak failure, it would be probably be the hammer spring (mainspring) and trigger spring, cylinder latch and cylinder latch plunger spring, pawl and pawl plunger and spring and possibly the ejector spring. Those parts are fairly small and do a lot of work during use so they are going to wear more.
For the Security Six, maybe, maybe the rear sight blade and maybe a front sight blade but if someone damages these parts something pretty drastic has happened or they just weren't being as careful as they should be. Do people drop guns? Sure, it happens. But if the front sight is getting beat up badly, what else is happening to that particular firearm and what do the owners other guns look like??