Your pan stamp looks like the 1966 p-glide format, pre Camaro. The format was based on model year, not calendar year the trans was built. But pans can easily be changed. Go to the Drivetrain Decoding section on the main page here, and scroll all the way down to see an example of where a powerglide could have been stamped. Not all got stamped in 67, but look around the entire leading edge of the bellhousing on both sides to include where it angles back, to see if any partial VIN stamps. Not easily done while it's in the car. Assuming it's not been altered, the absence of a partial VIN will only determine if 67 or earlier. I believe there is also date information cast into the aluminum case, however I'm not sure of the detail level it gives you.