Image 66622, 20.45 Northampton Castle Yard-Tunstead Sidings (6Z99, 30L), Northampton station car park

66622, 20.45 Northampton Castle Yard-Tunstead Sidings (6Z99, 30L), Northampton station car park
66622, 20.45 Northampton Castle Yard-Tunstead Sidings (6Z99, 30L), Northampton station car park 
 Taken through the access gates to Northampton's virtual quarry better known as Castle Yard the evening's empty wagons are seen being marshalled prior to departure by Freightliner's 66622. The 66 will work the 20.45 Northampton Castle Yard to Tunstead which carries a curious headcode of 6Z99; can anybody enlighten me as to why this is? 
 Keywords: 66622 20.45 Northampton Castle Yard-Tunstead Sidings (6Z99, 30L), Northampton station car park Freightliner
66622, 20.45 Northampton Castle Yard-Tunstead Sidings (6Z99, 30L), Northampton station car park 
 Taken through the access gates to Northampton's virtual quarry better known as Castle Yard the evening's empty wagons are seen being marshalled prior to departure by Freightliner's 66622. The 66 will work the 20.45 Northampton Castle Yard to Tunstead which carries a curious headcode of 6Z99; can anybody enlighten me as to why this is? 
 Keywords: 66622 20.45 Northampton Castle Yard-Tunstead Sidings (6Z99, 30L), Northampton station car park Freightliner

Taken through the access gates to Northampton's virtual quarry better

known as Castle Yard the evening's empty wagons are seen being marshalled prior to departure by Freightliner's 66622. The 66 will work the 20.45 Northampton Castle Yard to Tunstead which carries a curious headcode of 6Z99; can anybody enlighten me as to why this is?