The meaning of and standards for "specimen numbers"
In this case, your suspicion of year.month.day.sequential number is correct. The NHM specimen page has an image of the collector's notes:



On line 20, that specimen of Circus assimilis is accessioned, with a note on the right that it is the type for that species.

Generally however, there are no standards for specimen numbers in museums. Many museums that I have studied in have several different numbering or ID systems in use at the same time. Importantly, each specimen is accessioned under the system used at the present time. Sometimes these are later renumbered, sometimes not.

Even in the NHM, there are different schemes in use. Megalosaurus bucklandi, the first described species of dinosaur, is specimen PV OR 2482.

The NHM has 80,000,000 specimens. I'm sure that those doing the cataloging at the start couldn't have imagined the collection would grow to that size, so planning ahead for a single numbering scheme wouldn't even have occurred to them.