Net only falls into the 2 year rule if it is required to have a 27.1 net.

6:3 WINDOW NET
An SFI 27.1 ribbon-type or mesh-type window net is mandatory on any full-bodied car running 7.49 or quicker. For full-bodied cars running 7.50 to 9.99 or if vehicle runs 135 mph or faster, a ribbon-type or SFI mesh-type window net is mandatory unless otherwise specified by Class Requirements. SFI 27.1 window net, when required, must be updated at two-year intervals from the date of manufacture.............

Doug