The Troughton scale served as the first (de facto) standard of length in the United States, from 1832 until 1856. It is a measurement scale of 82 inches, subdivided to tenths of inches, on a silver inlay in a brass bar about 86 inches long (see here, p. 31). On p. 33 of the same reference, which dates to 1965, it is stated that the Troughton scale is located 'in a case on the right' relative to 'the Standards Vault at the National Bureau of Standards, Washington, D. C.'
The scale was brought to the United States and metrologically characterized by F. R. Hassler. (Another useful reference on these matters is History of standard weights and measures of the United States, published in 1905, esp. pp. 370 and 372.)
In 2004, a descendant of F. R. Hassler gave a presentation which is included in the NIST Special Publication 1068 ('Ferdinand Rudolph Hassler (1770-1843): A Twenty Year Retrospective, 1987-2007', available here). In the presentation, there is a picture whose caption reads (on p. 106, which is p. 130 of the NIST pdf), My father was Ferdinand Rudolph IV, on the left, with his brother, John, seated next to him. Here, they pose with some FRH's instruments that are now in the NIST Museum. True, the Troughton scale was not exactly one of F. R. Hassler's instruments, and in any case does not appear to be in that photo. Nevertheless, the NIST Museum would be a logical place for it. However, I was unable to find any record of it on the website of the museum.
Another logical place where it could be located is the Smithsonian, but I couldn't locate it there, either.
Finally, I did a lot of googling, including on google books. So far, the 1965 reference I mentioned above is the most recent reference I have been able to find that specifically mentions the location of the Troughton scale at the time of the writing.
It would, I think, be quite surprising if the Troughton scale is not in the NIST museum, and it would be very surprising if it has been lost or destroyed.
Can someone confirm the present location of the Troughton scale?