Hi Matt, I'm sure it isAdobe Garamond's Old Style Numerals.

I'm fairly sure - sorry Geoff - that it isn't Adobe Garamond. Although it's a close match to the printed piece, Adobe Garamond's upward-tilting top-stroke of the 5, and the unclosed bowls of the 6 and the 9 rule it out.

It is, I think, Peter Baker's Garamond-esque freeware font for medievalists, Junicode, or its predecessor JuniusModern, from the Old English Font Pack (which had the old-style numerals in default position, rather than as an opentype option, as in Junicode).

See this image of the relevant Junicode characters.