Cannot find serial number

I know it is a 1978 M900 90 hp inline six affectionately known as the Tower of Power series. My first ever Mercury. There is nothing on the bracket where most point to for getting the serial number. I poked around with the wonderful wrap around tin cover off but could not see anything but that timing tag which gave me the model number clue. In order to find parts etc. I will need the serial number which is unlike the Johnson Evinrude family. Truth be told, if you held a gun to my head and told be to get the serial number, I would just tell you to shoot me.

I am in the same boat with no motor serial.

I have a friend that has two Tower of Power motors he is swapping for some customers. He removed one motor and found the serial number on the front of tilt tube (the tube the steering shaft runs through). If you are looking down at the motor from above, as you would view it standing in the boat or leaning your head over the transom, you will not see it. I stuck my phone down in the well and as low as I could get it. Took pictures until the label showed in one of them. Hope this helps.