Hi Worktel - welcome to the forum! You've already heard lots of great advice - so I'll just add a few more points:
Notice in your Scott catalog - it does not list a price for 'used' - only mint. Your stamp is used. Which means if you actually had a used 375b here, it would be so rare that Scott has yet to even give it a value. Think about how likely that is!
Read your Scott catalog - especially in the front. Scott values are for VF stamps, and they give pictures to show what they mean by VF. Stamps that are lower quality than VF would not command the price listed in the catalog. Even VF examples rarely sell for full catalog value.
Here's a nice copy of 375b that sold recently on eBay
(this is a good example of what the color 'Lake' looks like...)
If you have a stamp for certification/grading, then PF/PSE/APEX are the top 3. But for an appraisal, you'll have a tough time. Actually your best bet is to post a picture here, and you'll get immediate, expert, and (free!) opinions. You can also get a good idea on your own by searching eBay
and/or other auction houses (Siegel for true rarities) to get an idea of what certain stamps sell for.
I agree that yours is a #332, and its value is minimal - around a dime...
When you see multiple values for a stamp - especially when one is minimal & one is high, always assume you have the lower valued item first - then you can start logically proving that you have the higher value item.
Don't forget - when you get your ID back from APS, post the result here!!