It turns out that the microfilm publication number is hidden (in this case) in the URL--that's underlinked in red in the screen shot below.
Admittedly, Ancestry.com indicates that this image came from roll 122 and contains passports in the 1864 time frame and with some searching I could back into which of all those NARA microfilm publications contains a roll 122 with passports from 1864.
But that, just like the URL approach, requires me to assume that I'm right.
And we know what assumptions can sometimes do.
It would be nice if in the "results" page shown as the first image in this blog post, Ancestry.com would include the NARA publication number and roll number.
It's not Ancestry.com's job to cite my sources for me. But, as a paying customer, I would appreciate it if they would make easier for the customer to cite theirs.
[images in this blog post were made on 27 February 2013]