175 mercury black max year

[The huntsman]

Sorry guys you have probably heard this a thousand time but I just bought a second hand mercury black max 175hp, apparently it has a 200hp power head on it. I can't seem to find a serial number on it anywhere! Its not on the welch plug at the top all that is on that is 175, I have the rego papers from the previous owner and it says the. Engine serial number is 123C5P88 and I found another number on it at the bottom right at the back of the motor on the block and it's 858 8631 I'm desperate to find out so if anyone can help that would be appreciated!!