The "pigeon grades" were often special order models from Browning, usually ordered by one of the bigger dealers since Browning would usually not make a special run unless at least 25 guns were ordered. Sometimes they were ordered as "presentation pieces" and given as gifts, etc.
Like others told you, it's the same internal components as the BPS made at the time, and they will have some significant engraving on the receiver, trigger guard, etc. and an upgrade of wood - but based on whatever the original order was. If you can get a serial number Browning can usually tell you when they were made.
I have a pigeon model - BT 100 - there were 200 made for a dealer in the midwest, each one separately serialized within the series of 200 ( mine happens to be # 90) - a lot of gold inlay, very pretty gun - but the one I have has very "plain" wood but that's how they were ordered. Very collectible, especially if unfired ....