30-31 front fenders are the same. But there were a few different styles. The eyebrow fender was early 30, the indentation that is a few inches above the light bar could stop a little before the front edge of fender or extend to the edge. After april 30 all fenders had a raised area around the front mounting screws.
Main difference is splash apron nose piece. On '30's its a seperate piece, welded to the fender. On '31's the nose piece is part of the splash apron.
Attached Truck photo is '30, attached Vicky photo is '31.
For the same time period cars, pickups, and AA's each shared the same style fender. So you could remove a car fender from mid 1930 and use it on a mid 1930 pickup or mid 1930 AA.
What are you thinking of doing? That might effect the advice you would be given.
The 31 style had the one piece running board splash apron and the nose bolted into the notch of the fender.
The 30 had the two piece running board splash apron and the nose was spot welded onto the fender.
Typically the 30 fender has a shorter flange where the apron was spot welded. The 31 was longer because they had bolt holes.
The fenders are basically the same for all production 30-31. There was the eyebrow fenders found in earlier 30's.
There were some subtle differences in fender construction and the following is not a complete list:
Some 30 fenders have the flat area over the frame with a rolled over edge.
For the 31 style there is some differences in how the crease that run the length of the fender terminates at the rear. One type will have a bit more of a convex upward curve as you get near the running board flange. While you might want to pay attention to keeping the fenders the same, there is a known original car with both types of 31 fenders.
That is the quick overview as found in my brain today. Hope that helps.
Note also that '30 pickups did not come in with the cars... Cars changed from model 29 to 30 in Jan 30; trucks and other comm As did not become 30 models until June 30. Just got a May 30 pickup and a lot of it is still 29 model stuff. Given the 3 month, "use what we have" policy and the early 30s could have a number of things on them.