When I ordered a spring tie bolt from one of the parts dealers it turned out to be one of these modern things with threads all the way to the head. Of course the local stores were useless for finding anything close to the original bolt. Even the so-called hardware store has no square-head bolts at all.
My uncle, Charles Miller, who died in 1960, came to the rescue by leaving behind a large wooden box of old time hardware.
This isn't an exact match for the original, but I'm declaring it close enough. Old hardware is one of the reasons I go to farm auctions and estate sales.
Bob Bergstadt makes the correct square, extra thick head bolts for the front and rear springs.
I thought so too, but these don't look like tall heads.
It is important to get a very strong bolt for that application. I think you should go with Bob. You can use your uncle's bolts for other areas which need less strength.
If they're like the photos, my old one may be just as good. It appears to me that this bolt is primarily for leaf alignment during assembly, and that it's the clips (much stouter) which take the stress of holding the leaves together.
The photos on Bob's website are old. Here is a picture of his current offerings:
Great story Steve...made me think of my Grandfather.
Now, that's more like it. Bob needs those photos on his website.