O2 sensor 2001 or 2003???

I'm having a problem identifying the year of my engine. The sticker says 2001 sx250txrz but the O2 sensor block that is in the engine looks to be from a 2003 sx250txrb. In 01 the sensor block is flat and seals with just a gasket in 03 it seals with o-rings and a gasket. Here is where I can't get an answer from anyone with any certainty. Can an 2003 sensor block be put into an 2001 engine block? My problem is I bought the boat and motor used and I don't know what was done. Basically what I need to know is what the area behind the sensor block on an 2001 engine looks like, is it flat or could someone put an 2003 sensor block in there and have it not work properly?

[boats.net]

The O2 sensor block can't be changed it is the engine block. It may have had the powerhead replaced which would supersede to newer block. Check with previous owner to see what has been done to it.

Thanks

Let me ask this question. Can a Yamaha engine block year be identified by the numbers stamped in the casting? I can see them and read them but they don't mean a thing to me.
I can't get a hold of the last owner to find out if they re-powered the motor.

[boats.net]

No the numbers wont tell you anything. The O2 sensor mounting is the only change that was made to block.