Thanks Everyone,
Jeff - my bolts look like the one in your photo - only mine are completely gloss black rather than the nice rust colour pictured :-)
The bolt threads were left untouched, so over time a small rust stain crept up around each bolt head. In the past, I removed each bolt, treated with WD-40. This seems to have stopped this problem.
So what I hearing is that I should stick with my original bolts, they should be blasted clean, and then gray/black color phosphate and oil finish added (including the threads) - can someone confirm ?
Thanks again everyone !