No, not poor winterization. That will leave water in the bottom end, which can crack your case halves. I'm talking about water that was ingested, and then left to sit, causing the crank main bearings to fail. As the balls disintegrate, the pieces they shed cause that sandblast look. On mine, it also slowly caused the spark plug gap in one cylinder to close up, too.

I could be wrong. For your sake I hope so, that's about as bad as it gets for these motors.

Regardless, you should split the motor and replace all the gaskets and crank seals. While you're in there, you can check the crank bearings; it'll be painfully obvious what is causing this. This motor is 24 years old.

The winterization issue that I'm referring to is that cooling water in the pre 94 models was passed thru a small loop in the bottom end of the block. This water had to go uphill to exit the motor, so just bumping the engine a few times wasn't going to remove it.

Here's another bad thought. How much you wanna bet one of the previous owners didn't do this, and that passage is already cracked, causing all kinds of weird running issues, as water makes its way into the bottom end? Ugh.

As previously mentioned, you can simply remove this portion of the cooling system; it isn't needed. If you leave it, you'll need to ensure you get some RV antifreeze down there each winter, so you don't split your crankcase.

Again, get an OEM service manual. It's explained very well in there. See the below attached litho of the crankcase of a 1993 SL750. Note the hose, #17, and pipe, #15. These are the ones that were eliminated in the 94 and up models.