Well,
The verdict is in:msp_scared:
I guess ya win some and ya loose some.
This saw came today and it definitely has the 4000 top cover, and muffler.
But it also has the 3400 flywheel and recoil starter....the pulley of which was broken but it was still working just as it should.
I pulled the muffler and the p/c are VERY clean and the piston had THIN rings!!!
Got my calipers out and using the depth gauge, measured from the back of the cylinder to the front of the exhaust port............80mm.
Went out to the shop and pulled the muffler on one of my 3400s, measuring the same manner....................75.5!
I don't have a 3700 to measure so I Can't Be 100% certain but I feel real confident that this saw is indeed, a 4000!!!
It shows 155+ on my el cheapo compression tester, put a little fuel in it and less than 10 pulls later it kicked off and ran like a champ.
I figured fuel lines and a carb kit at a minimum but it looks like I may have gotten lucky.
I dressed the bar rails as they were in pretty sad shape and put the(Stihl in about 70%) chain on.
Looks like I may have come out alright on this one!
Mike