January L35 and March L78, so early L35's would have the purple buttons and L78's should have the Pink (salmon). I have know idea when the L35 transition to the pink (salmon) buttons took place or if early L78'S could have the purple buttons. Revision dates do not reflect when actual production started but early L35's should have purple buttons.
Was the dipstick installed when the motor was assembled or when the car was. I'm thinking it was when the motor was assembled.
My earliest motor, stamped 0214MZ, has the pink (salmon) stick. Based on some of your posts about date changes then it may be early Feb the color changed from purple (?) to pink.
As some extra info about oil pans I came across this from the Chevelle group (I assume the 396 Chevelle has the same pan):http://www.chevellestuff.net/tech/articles/oilpan/index.htm