1927 Packard

Hello,

Can someone tell me what the externally visual differences are on a
Packard Six engine --- 1927 4th series versus 1926 3rd series?

The internal differences are great --- larger bore, high compression
head, etc. But if you've got one sitting there, how do you know
(aside from serial number, which can be changed)?

I'm going to look at a car that has a replaced engine. How do I know
if the replaced engine is a 1927 or an earlier (or later) engine?

--Scott
Troy, NY