Just recording this for posterity.
Here are some BD.212 bolts with flat and internal star washers for the chrome moldings on the bonnets of SS, Mark IV and Mark V.
The size is #3BA x 3/8" long. The head is ground off into a pent roof shape so it will slide into the groove in the chrome.
They were well protected from rust under the bonnet, so fortunately none of mine broke and I didn’t have to make any.
The BD.2248 bolt for the step bead center chrome strip on Mark V is also #3BA thread with the same pent roof head, but 9/16" long, because it has to go through an extra 1/4" of rubber. All of mine were rusted solid and broke, so I am making all new ones out of #8-32 x 5/8" UNC screws.