On a 10 bolt gm rear end how to deciper the stamping numbers to tell what the gear ratio is the tube on the front passenger side reads BP 1204G1?

To decipher the stamping numbers on a 10-bolt GM rear end, you'll need to focus on the code stamped on the housing. The "BP" indicates the plant code, while "1204" is typically the Julian date of manufacture (the 120th day of the year). The "G1" can indicate the gear ratio. For GM 10-bolt rear ends, you can refer to a specific gear ratio chart for that particular plant to determine the gear ratio; in many cases, the gear ratio can also be found on the ring gear itself, stamped on the edge.