http://www.safetydevices.com/motorsport/technical/
Have a read up of this .
Rules do change , but it used to be that , If the cage complied with the original design & had the correct no. on it , then it was OK for a Historic spec car .
S D should still have details of original , are ,[ I have found ] usually very helpful , & , [ unless they have moved in the last few years ] , only just down the road from Cambridge .