The TAC has released an updated alert on steering arms for early Fords, outlining some particular brands which are now known to be cast iron. While identifying specific brands and materials can be difficult, there are some suggestions included in the Advice Sheet.
The underlying point is that if you know or believe that your car to have one of the brands of steering arms as outlined in the bulletin, please do not drive it as failure can be sudden and potentially catastrophic.
You can review and/or download TAC Advice Sheet 2:20 at the TAC Bulletins page on the ASRF website. Further enquiries should be directed to your divisional TAC representatives.