The left-hand number, 12, before the colon, is the catalogue serial number from the published PUL catalogues of 1932 and 1941. Since these numbers start again at 1 for the different subject sections and appendices of the catalogue, the same number may occur more than once.
After the colon comes the library's accession number(s) for the manuscript(s) corresponding to the catalogue number, 7276 in this case. The accession numbers are clickable links that take you to the full information for that manuscript.
This browse returns 278 manuscripts with 266 different catalogue numbers: