Parker Single Barrel Trap value
 

My dad left me a Parker Single Barrel Trap with 32" barrel, Grade C MFG in 1920. It is in good condition and I was wondering what the approx value might be.

We'll need to see several high resolution pictures in order to suggest a value.

The gun that you have is the most common configuration that the single barrel trap was made in. But there were still not a lot of them made. As a general guide, it is usually hard to touch one for under $2,000 - $3,000 that has not been seriously messed with. But, a lot of them have been seriously messed with by having their stocks altered by multiple owners.
As Dean said, photos are the only way to even start to tell what type of condition the gun is in.