I bought a Stihl ms 290 on Facebook but I believe it is a MS 261, MS 271, or MS 291 because the parts number is 1141. The serial number is 503550148.
Can you take pictures and post them of the saw?
I am leaning to believe it is probably most likely a 291, though the 271 and 291 are almost identical except the 291 runs a 47mm piston and the 271 runs a 44.7mm piston. Without pulling the cylinder off, we won't know what you have there. The 261/271/291 are all in the same family of 1141, though if you look at part numbers, they are different in the end number between the 261 and others:
* MS 261 Crankcase fan side: 1141 020 2612
* MS 271 Crankcase fan side: 1141 020 3019
* MS 291 Crankcase fan side: 1141 020 3019
MS 290s look to start with 1127. I wish I had a way to look up your serial number, someone else on here might be able to. I have two 261s to look at and one starts with 528 and the other is 535, I also have two 271s and they start with 521 and 533....so not much help there.
If it has not been altered, you can look at the chain sprocket, if it has a rim sprocket, then probably a 261, if a spur sprocket then most likely a 271/291.
A 261 can also have a metal clutchcover with captured bar nuts and a decomp valve on the cylinder.
It will be a bummer if it ends up being a 271 instead of a 291 only for a bit of horsepower difference, but the 271 is a good saw, I like mine. The only think I have changed on it was trading out the spur sprocket for a rim sprocket for quicker swaps down the road. I rebuilt a 291 for friend and he is really happy with that saw the only bummer with the 271 and 291 them is the lack of aftermarket cylinders.