A quarry worker at Mepal, Cambridgeshire, found the remains on Monday.
It is thought the head came from a site in Bedfordshire, 40 miles away.
Cambridgeshire Police had previously said they were not currently connecting the discovery to any missing persons.
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"A post-mortem examination has come back inconclusive," a spokeswoman said.
"Specialists are still unable to identify whether the head belongs to a man or a woman, however, based on the information available at this time, it is believed the person died before 1 January 2015.
"Detectives from the Beds, Cambs and Herts Major Crime Unit (MCU) continue to work with specialists in order to establish the gender and identity of the deceased."
She added: "We are not linking it to any other investigations, however we are keeping an open mind, and we are in communication with the National Crime Agency."
In particular, they said they were not linking it to missing women Natalie Hemming and children's author Helen Bailey - both of whom disappeared in recent weeks.
The force also said the find had not been linked to the murder of Ramute Butkiene, whose torso was found inside a suitcase in her killer's flat last year.
The head, which police said was not a skull, was found by a worker at the Mick George site at 15:00 BST on Monday.
Police remain at Mepal and the site in Bedfordshire. Nothing further has been found at either site.
Natalie Hemming, 31, of Milton Keynes, has not been seen for more than two weeks. Her partner, Paul Hemming 42, has been charged with murder.
Children's author Helen Bailey, 51, of Royston, Hertfordshire, disappeared while walking her dog on 11 April.
The head of 42-year-old Ramute Bukiene has never been found. Her boyfriend Vytautas Jokubauskas, of Peterborough, was jailed for her murder last year.
A number of calls have been received from members of the public in connection with the investigation, police said.
The Mepal site where the head was found is used for landfill and recycling, as well as the supply of aggregates such as limestone and gravel, and mixing concrete.
Tests on a severed human head found at a quarry in Cambridgeshire have failed to identify the gender but revealed the victim probably died before 2015.