According to http://www.nastyz28.com/camaro/zsusp.html
"PH" was a 3.55:1 ratio locking diff.
Used in '67 / '68 according to the drop-down
"G1566" = July 15, 1966? (Not sure about that)
Rear Housing Casting Date
Example: D141 - (D = Month, 14 = Day, 1 = Last digit of year (1971)).
Month: A = Jan, B = Feb, C = Mar, D = Apr, E = May, F = Jun G = July, H = Aug, I = Sep, J = Oct, K = Nov, L = Dec
has an "axle de-coder" that confirms the ratio, and says it was built on Aug 20.
has differing information.
IMO, the safest way to correctly identify the gear ratio and carrier type is to remove the cover and have a look. Divide the number of teeth on the crown gear by the number of teeth on the pinion. There are tons of places on the internet that will identify the axle size/family visually by the number of bolts on the cover, etc. Richmond Gear for one.