Depends on when it was made. Sometimes it describes the variant.
If you mean Citori, 100-1000 USD
Magnum
Part of the serial number.
Part of the sn that tells you when made. In this cased 1992.
The serial number of the weapon.
Nothing special.
You have a serial number of 00000 the letters NW mean your BPS was made in 1994.The B52 could be referring to the model number.
If you mean a Citori; 100-5000 USD
part of the sn
Your serial number with the prefix of 4G indicates that your Browning light weight 12 gauge shotgun was made in the year 1964.
Go to Browning's site, click on Support then date your firearm.
Grade I simply means the Standard model which usually has less engraving and ordinary wood grain in the stock. The higher the grade, the more ornate and / or hand work went into making the gun.