The fonts in the Univers typeface family were assigned numbers by its designer, Adrien Frutiger. (You can see the scheme on, for example, page 29 of The Art of Typo.icon.ography, by Martin Solomon.)
The names given here have to be prefixed with a source letter to actually be usable. Since my purpose here was just to demonstrate the correspondence between typeface variations and the naming scheme, I left the source out.
45 (light)
46 (light italic)
47 (light condensed)
48 (light condensed italic)
49 (light extra condensed)
53 (medium extended)
55 (medium)
56 (medium italic)
57 (medium condensed)
58 (medium condensed italic)
59 (medium extra condensed)
63 (demibold extended)
65 (demibold)
66 (demibold italic)
67 (demibold condensed)
68 (demibold condensed italic)
73 (bold extended)
75 (bold)
76 (bold italic)
83 (extra bold extended)