1972
Thanks doc! 
Just to let you know, this could easily be a 1968 or 1969 model as well. 
1968 was the first year of the plastic Butterfly. I thought (could be wrong) that in 1968 they were using the Butterfly sticker on the cap (or screen print), like this one:
It’s tough to say exactly, with 100% certainty the exact year of that Butterfly. Those were made from the mid 70’s through the early 80’s. Late 70’s and early 80’s are a little easier to tell. They began to coat the axles with a material in the late 70’s (78 or 79). Those usually aren’t rusted out, like the earlier ones get.
Not to disagree with Rick (from the post Doc made), but I always thought that version was mid 70’s, around 1975-1980. The progression I was told was: Wooden, Plastic with stickered cap, plastic with embossed small butterfly logo, plastic with large embossed butterfly, translucent butterfly in the early mid 80’s.
Well; I’m certainly no yo yo Historian, lol.
Here are a few pics from Luckys’ Book>
Was thinking about pulling out Lucky’s book when I got home as well ;D
The decal/silk screen version must’ve just been a year or two. Then the Large stamp.
Does it say, was the small stamp prior to the bright colored version? Maybe I had it all backward. Maybe the small stamp was first, colored silk screen, then large stamp
Really digging those Butterflies you guys are showing off!
Here is my little Butterfly collection
Now I have to learn how to shoot and add photos to the discussion so that I may show off my Butterfly collection.
I really like the green with orange caps. Seems like combinations of yellow white and orange are more common from that era.
Love the collection! I have this one. Just put a fresh kitty fat on it and gave it some love. Very satisfying. Thanks for the suggestion.
Is there a way to get rid of the rusty spots on the axel so it doesn’t chew string really fast? Could I take some type of chord and rub it over the axel enough to lessen the spots?
I don’t know if theres any easier way, but I had some success using a thin picture hanging wire on mine. Just wrapped it around once and pulled it back and forth until I got most of it.
Okay, thanks! I will either try this or try to find something similar
The green one I believe is very early 80’s, prior to the translucent ones. All the one’s I’ve seen from the 60’s-70’s have been white/yellow/red in different combinations.