At some point Apple switched from AMI6820 to Synertek 6520.
Was reviewing some pics and Mike's registry, this seems to be very common for NTI board that we know we're populated. Since they all seem to have the same date codes for the 6520, a good assumption is they are original chips. Remember a few NTI boards like Postman didn't have chips till they were restored so we can't use them as a guide.
So this means if you are building an NTI replica,it is perfectly valid to use a Synertek 6520, way easier to find than a plastic AMI 6820 with the raised logo in a triangle formation (I have actually never found one in the wild).