ok so i am looking at buying a used acog but i want to know a couple things

1. whats the difference between the ones with the fiber rod on the top, and ones without. i think the t01sn is one i saw without, is it newer ?

2. about how long is an acog good for before needing tridium replaced?

3. any other tips for buying a used acog, im looking for a 3-4 power one for my AR?

1. The fiber optic rod is for daytime illumination.

2. About 12 years.

3. I got a good deal on a used one from SWFA several years ago.