Bruce Smith wrote:

No Type 11 drawings there, but there is a Type 7 drawing, and no sign of any anchor in that drawing. The next page shows the now infamous USRA design - sure enough the anchors are shown, as they are also shown on the Type 21 drawing. I'm beginning to think that the anchors were an improvement to the design added around the time the USRA car was designed (1919) and therefore weren't present on the Type 11 or maybe even the Type 17...

Bruce, the center anchor was invented in1903 and became widespread by 1910. It happens to have been invented by a name well-known in tank car history, Union Tank's John Van Dyke, and was the basis for UTL's X-class tank cars. It was patented originally, however, and at least in its original form, may have been hindered in its spread by the need to pay licensing fees to UTL.
I can't tell you when the first AC&F designs had center anchors. But SP purchased tank cars in 1913 from Pressed Steel Car Co. which had center anchors. Unfortunately, their previous purchase of tank cars was in 1908 (without center anchors), so I can't narrow it down much.

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