Model number (on the back cover):
A1876 on the iPad Pro
A2014, A1895 on the iPad Pro Wi-Fi + Cellular

Model number (on the back cover):
A1980 on the iPad Pro
A2013, A1934 on the iPad Pro Wi-Fi + Cellular
A1979 on the iPad Pro Wi-Fi + Cellular (China mainland only)
 

10.5" Retina display
Model number (on back cover):
A2152 on iPad Air (3rd generation)
A2123, A2153 on iPad Air (3rd generation) Wi-Fi + Cellular

Model number (on back cover):
A2133 on iPad mini (5th generation)
A2124, A2126 on iPad mini (5th generation) Wi-Fi + Cellular
A2125 on iPad mini (5th generation) 

Model number (on the back cover): 
A1893 on the iPad (6th generation) Wi-Fi
A1954 on the iPad (6th generation) Wi-Fi + Cellular

iPad (7th generation)
Capacity: 32 GB, 128 GB
Model number (on the back cover): 
A2197 on the iPad (7th generation) Wi-Fi
A2200, A2198 on the iPad (7th generation) Wi-Fi + Cellular

iPad (8th generation)
Year: 2020
Capacity: 32 GB, 128 GB
Model numbers (on the back cover):
A2270 on iPad (8th generation)
A2428, A2429, A2430 on iPad (8th generation) Wi-Fi + Cellular
White or black front bezel

iPad Pro 12.9-inch (4th generation) 

Capacity: 128 GB, 256 GB, 512 GB, 1TB
Model number (on the back cover):
A2229 on the iPad Pro
A2069, A2232 on the iPad Pro Wi-Fi + Cellular
A2233 on the iPad Pro Wi-Fi + Cellular (China mainland only)

iPad Pro  11-inch (2nd generation)

Capacity: 128 GB, 256 GB, 512 GB, 1TB
Model number (on the back cover):
A2228 on the iPad Pro
A2068, A2230 on the iPad Pro Wi-Fi + Cellular
A2231 on the iPad Pro Wi-Fi + Cellular (China mainland only)