The mag looks just like my phosphated 12 rd Mec-Gar mags. Says Sig Sauer 229 .40/.357 on the left side and has marked witness holes on the right. The body and follower look the same as my other mags. The one different thing is that it has a non date clock baseplate like what are on the 229-1 9mm mags. Any idea who made this mag?

For those P229 and P226, if the mag's not marked Made In Italy, you can almost assure yourself that it's Checkmate. I have P229 mags NIW from SIG, with "Made In The United States" on the bag, but the mag has no country markings on it.
US made mags = Checkmate, unless SIG has an additional US mag maker we're not aware of.