Extra Order Tenders

While working on re-coating the inside of SS&S #6's tender tank (an 1891 3' gauge Baldwin 2-6-0), we stumbled across a couple of rivet holes that had been plug welded a long time ago. They are in exactly the right place for an Extra Order tank builder's plate judging from pictures of other engines with extra order tanks- this is not surprising, as we've known for some time that it has an extra order tender tank, we just have practically no information about it.

The discovery of the Extra Order builder's plate rivet holes was followed by the discovery of a stamp found on the right side water leg close to the top of the grab iron. The stamp reads 13-530 (I believe). I'm not sure if this has any relation to the extra order tank order.

If anyone has any information about what this stamp means or any other information about its Extra Order tank, I'd love to hear it.

Ultimately, I'd like to figure out what number and date is supposed to be on the plate that should be on the tender. I went to DeGolyer's online extra order databases to see if there's any record of 6's XO tender, but they don't have any records new enough. We do know for sure that SS&S #4 (6's sister, but from 1888) got an Extra Order tender tank in May 1914, and the XO plate number is 16584. The 6 got its Extra Order tender tank before being sold to Argent Lumber Company in October 1927 judging from early pictures of 6 at Argent.

Thanks everyone!