I am looking at a used P228 for sale. It is listed as a W German gun. (Although slide only says Germany not W Germany) On the right side of the slide where the 2nd serial number should be has been milled out. Additionally on the left side of the gun on the frame above the trigger is another serial number. The serial numbers are the same. But the right side slide has a space between the first three and last three numbers. The left side does not have the space.

I have owned several Sigs over the years. I'm presently down to only one Sig a P220ST. I am curious about the for sale gun. Maybe a replacement slide? Or maybe an ex police gun? I know NJSP milled out their logo on their traded in P7's. Seller has not responded. Any ideas?

Yes, replacement slide with the milled out serial number.

As to the serial number on the frame's left side, it was most likely a LE gun, and that number was done aftermarket by the LE agency.

So, this is a FrankenP228.

Here is an example of a FrankenP228 (with slide having the serial number milled out) that I used to own. Frame is original M11-A1. The slide was built (not made, obviously) for me at SIG NH.