Rick,
The Cross and Cockade Dolphin book has numerous references to "tubular steel' cabane struts on the production version but I couldn't find a definite statement on the interplane main struts. It also mentions that the wood (ash) undercarriage struts on the early prototypes were replaced by tubular steel on the production versions.
The section on the restoration of C3988 however illustrates new carved ash wing struts used in the project. I'd say they were wood as shown in the book and often painted in that common "Battleship Grey" we see on other Bristish aircraft of the period.
Hope this helps.
Cheers,
Lance