Hi Anne
 
A system similar to those already described is used at Tate: loans-in
and acquisition candidates are given an 'X' pre-fixed object number and
assigned to their own departments ('Acquisition Candidates' and
'Exhibition and Display Loans In'). When an acquisition candidate is
accessioned it's given a new accession number, with the old, 'X' number,
retained as an alternate number and the department is changed to 'Main
Collection'.  
 
Jon Thristan
Tate