Keep in mind that serial numbers went in production batches, so where one production run ends of say, P-51D-5, the next serial number could be a B-24 or something else. But the first run of P-51D-10 were as follows:
44-14053 through 44-14852
North American P-51D-10-NA Mustang
c/n 109-27686 through 109-28485.
Most likely, any P-51s with higher serials than that would be -10-NAs or higher revisions.