I'm no expert at all... but I do love a google challenge.
From my reading of various blogs and articles, and several images, in addition to what's already been posted in this thread, it seems to me that Chanel was using the interlocking C's for a long time prior to Karl Lagerfeld's leadership in the 1980s. However, the original Chanel handbags she launched in 1955 did not have the interlocking C's as a clasp - they had a plain sort of clasp. Lagerfeld changed the clasp to the interlocking C's in the 1980s.
If you look at the wikipedia article on Chanel, it states that the interlocking C's were first trademarked in the US in 1924 and they reference the original trademark registration here:
http://www.trademarkia.com/cc-71205469.html
For instance, you can see the interlocking Cs in this 1938 ad for Chanel perfumes (from the blog "The Painted Woman" - scroll down and click on image for full size):
http://thepaintedwoman.blogspot.com/2009/12/christmas-shopping-1930s.html
This blog shows a comparison of an original "2.55" Chanel bag (named after its release in the 2nd month of 1955) with a contemporary classic bag and you can see the original clasp design:
http://fashionpearlsofwisdom.blogspot.com/2009/11/chanel-255-handbag-history-facts-and.html
and here are a couple Chanel bags dated to the 1960s:
[edited - when I read it closely, this was actually a link to the blog of a non-VFG store and so I can't post it here, I think.]
So in a way, both opinions are correct: Chanel
bag clasps became interlocking C's in the 1980s, while before that they were plain. However, Chanel's logo of the interlocking C's has been around ever since the 1920s (and I think if she had put any C's on her handbags, she would have used her trademark interlocking ones, not side by side C's).
I hope this is helpful.
Jen