sun gauge questions

Anyone know how to date code sun gauges? I know how to decode SW's but i'm wondering if there is a way to do the same with sun gauges.

Any help,pics,or links would be greatly appreciated.Thanx

 

all my older sun gauges had a date silk screened onto the back of them, my blue line ones were all from the early to mid 70s.

 

Thanx drptop70ss,Yah most of the blue lines I've seen and the one's for sale are from the early 70's.Which is perfect.
There are also some black face standard,SW looking ones that I am considering purchasing just need to make sure of the dates.The guy says they are from the late 60's/early 70's but he cannot confirm.