Have you had a look at the bonnet flange? It should give the chassis number and the factory clock number of the guy who made the bonnets. I can’t find my chassis number on the crossmembers of my original chassised (?) 1969 4/4 either! Good luck!
1969 4/4 1995 plus 8 1996 Plus 4 4seater 2002 Aero S1 2013 M3W 2014 Plus 8