More serial number problems

I'm really confused. I bought a mercury 500 recently that was purported to be a 1981 model, but according to the serial number it's a 1977---I think.this particular motor needed a new starter solenoid (it was absent when i got it) and according to the serial number, a new one mounts on the starter by means of a circular bracket. That's not the way it is however. The solenoid this motor needs mounts flat on the upper left front of the engine on a piece covering the carburetors. After checking various sites, some of the say it requires the solenoid with the circular bracket, and some indicate one with a flat bracket is needed. Clearly the flat bracket is needed and I believe the solenoid I need is part #852565. I need to know what engine I have and would appreciate some help determining what this thing really is. The serial number on this engine is 4577631 which according to some means it is a 1977. Can anyone help me with this dilemma?

Post pictures of this motor.----And solenoid can be mounted anywhere on a motor and it will do the job.

Motors i am ok with. Computers not so much. I have tried attaching photos, but can't seem to get them to go through. I could send them via private message perhaps if i can figure out how to do that.

The graphics on the motor pictured show it to be a 76 / 77 model.

That's what i came up with too, but the starter solenoid has the newer mounting arrangement. I tried to include photos of the mounting surface, but couldn't get them to go through. The photo shown is the only one i could get to go, and it is the worst of the bunch. The 1978 model has the same kind of mounting arrangement as is on mine, but it shouldn't.

The only reason for inquiry is to clarify what engine i have for future pats needs. I guess i'll just have to look closely at the original parts and match them up as best as i can. Anyhow, i appreciate the help in pinning this down.

perhaps it has a newer powerhead installed on it.

That's possible. Is there a serial number on the block i can check and if so where would i look?

http://www.ebay.com/itm/OEM-Mercury...ash=item41a1c0ad0d:g:DW8AAOSw3KFWcKyX&vxp=mtr
Is this what your looking for?

That is a 1977 serial number.

Well, I guess we're all agreed,it's a 1977 all right, or maybe a mixture. I was able to run down a solenoid which is really for a 1978 and newer, but it fit perfectly. It ran $17.00 while to one shown for a 1977 goes for $127.00--- if you can get one. Good news in the short run, we'll have to see what the long run brings.

Thanks for the help.