Request Info on Parker 20 Gauge

[Jay M. Siegel]

I have a 20 gauge Parker hammerless double-barrel shotgun with some markings that I cannot identify. Any help would be most welcome.

The serial number (220922) appears in three places: on the side of the barrel lug; on the watertable; and on the inside of the under-barrel left hand rest. The gun was manufactured in 1927.

The frame size, shown on the barrel lug, is 0. Above the 0 is the numeral 20.

The type of steel shown on the barrel rib is PARKER STEEL.

Question 1. Above the serial number on the watertable are the grade letters PH. Below the serial number is the single letter P. What does this mean?

Question 2. The weight of the barrel shown on the flats is 3. What does this mean?

Question 3. In addition to the number 3 shown on the flats, the other markings on the flats are:
HT
A
The letter P inside the curve of what looks like a tall numeral 2 with a shortened tail.

What do these mean?

Question 4. On both sides of the barrel lug are the words:

OVERLOAD
PARKER BROS
PROVED

The words OVERLOAD and PROVED are arced above and below PARKER BROS.
What does this means?

Question 5. I may be interested in selling this gun. Where can I get an accurate appraisal of its value?

Thanks to anyone who can help.