Welcome Lonedan!

Scott's Catalogue lists values for used PNC's. You can check out your local library for that. You can also order Hebert's PNC values Catalog from stampwants.com or from the APNSS (American Plate Number Single Society)-- Runs about $28 for Hebert's. I just ordered one last week. Generally speaking, any plate numbered coils on cover usually have a higher value than just the stamp. You were wise to keep those! I recently saw a "Toledo Brown " on cover on
ebay selling for starting price of $999! Blows my mind!
Remember that condition plays a big part in the value!
Also, check out:
https://goscf.com/t/2287#2287Gussyboy1