Full face helmet mandatory all cars 9.99 (1/4) or quicker.

"For all 9.99 and quicker closed bodied cars, a full faced Snell M2010, M2015, M2020, SA2010, SA2015, or SA2020 helmet mandatory; shield permitted."

Generally, helmets go out of date 12 years after the sticker date. Personally I wouldn't use an MXXXX helmet.

If the results don't match the theory, change the theory.