Although, the cars themselves may have been assembled in the early 1900s, I believe Phillip Marshall suggested that the tanks themselves may have actually belonged to earlier, standard gauge tank cars.
Yes, thank you Steve.
The casting on the top of the dome of the larger tank (I don't know about the smaller one) is marked "Harrisburg Foundry & Machine Co. Makers Harrisburg Pa 1878". Also known as the Harrisburg Car Works, this company was the first major builder of railroad tank cars during the Pennsylvania petroleum boom of the 1860s-1870s. They went out of business around 1893.
https://www.midcontinent.org/rollingstock/builders/harrisburg1.htmThe larger tank bears a striking resemblance to an 1875 Harrisburg tank car design for the PRR illustrated at the link above.
I'm quite certain the larger tank is an 1870s standard gauge tank car that was acquired second-hand by Standard Oil and the B&SR and converted to narrow gauge.
A discussion of Harrisburg tank cars (including ours) on RYPN several years ago found at least four still in existence, of which ours is apparently the oldest based on the dome casting date:
http://www.rypn.org/forums/viewtopic.php?f=1&t=34090&hilit=harrisburg+carsThere are very few Harrisburg tank cars left, and very few freight cars from the 1870s left in general, so it's a significant artifact even without the narrow gauge part of its history.