Thankyou all for your information, especially Anne.
I have found a no. on the wooden [ATTACH]189043[/ATTACH]base and it corresponds to one of the numbers on the boss: 200237. The rust has eaten into the metal coating and destroyed the metal surface. I doubt if cleaning the rust will reveal the lost dwg no.
I also realised under closer examination one of the hand engraved nos reads Dia 10′ 9″ which must be the overall dia of the 4 blades when fitted. The actual height of mine from base to tip is 5′.
I have attached a photo, or rather being new to this forum I hope I have!!