Mk and extras
Mk1 small black turrets £250 - £350
Mk2 small green turrets £350 - £450
Mk3 Tall green turrets £ 425 - £500ish
Nato Ret for me is the better of the two and A good Mk1 is better than the rest in my opinion.
Mk1's are getting on a bit so a Mk2 is the safer bet , there was an horrific batch of Mk3's that had very close ranges between 35 and 55 yrds
Try before you buy is a must and if needed get a friend to look through it for you.
On the whole money well spent , they hold there price second hand.