What is the thread size for flangenuts of protection cover?

During operation the chainsaw MacCulloch type MacCat 2.3 ci (38cc) with 18"blade - Mod 60016017 SER 11-012421
I lost one (1) hex. flangenut of the protection cover .
I know the size of the thread which is 1/4 " but now the question :
What is the exact type of the thread ? (BSP-UNF or other? )
Where can these nuts be ordered ?

wim Muller
August 2009