I'm sorry to have to ask but I am once again stumped.

Picked up a 383, '66 casting date machined block, crank, etc. that someone had never gotten around to assembling.

Looking at flat tappet hydraulic lifters and find some, but not all, make a distinction on year; many list '68 up, but some don't even mention it.

Is there actually a difference in lifters between 67 back and 68 up?

Thanks in advance!