The bolts on my C15A were also missing so I can't answer directly.
In the picture on the left, it looks as if the bolts may be threaded into the casting and have either sheared or been neatly cut at the face of the casting. You may be able to soak them with penetrating oil for a while (days, not minutes) and then weld a nut to what remains and get the stubs of the bolts to back out. The heat of the welding seems to act to help release the rust in the threads. With the stubs out, you should be able to measure the threads.
This part is only a suggestion, I have no proof it is correct. If the holes in the bumper look to be in good shape and round, perhaps they were made a bit oversize to allow some adjustment of the bumper positioning?
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