I have a holley 3310, with #3878261-EH, date 015. Anyone know what it's
for? Or a good holley id site?

http://www.tocmp.com/manuals/Carbs/Holley/MasterList/index.html
65 chevelle 396 425hp

Good job - thanks

The 3310 was the Holley for the '65 Z16 Chevelle as swerven already posted, but it was then also sold by GM at the parts counter for many years after '65.
The date code on this carb 015 says it was sold over the parts counter.
0=1970
1=January
5=5th week

Oops. Dyslexia got me that time. Saw that five on the end and figured it was 1965.