It sure does look like a nicely homemade die. Seems like if it was made by a commercial firm there would be some kind of logo, name, or other identifier on it. Easily within the capability of someone with access to a small lathe and horizontal mill. Also looks like it is made from allthread rod of some type. No reason to thread all of the OD, what are you going to hang on to when you thread it? Don't think a commercial maker would use allthread or an old bolt but a hobbyist/home machinist might.
Just my guess, I could easily be wrong, not trying to diminish the utility or quality of it.
Picture of the top
Picture of the interior. Opening is .474" and a crimp as tight as .468+" can be applied. I reckon the through hole is to allow air to escape. Don't know why the thread tops look round, but they are not.
Though crude, there is something about it that says it's not a one-off.
Likely someone wanted a separate taper-crimp die to add to their three-die set and whipped one up to order. The threads look like they were cut with a die instead bring single-pointed on a lathe or roll-forged, maybe someone had a big threading die handy or turned and threaded it on their lunch break at the Navy machine shop or any number of other scenarios. Maybe it was someone's high-school shop project demonstrating accurate thread form/fit, knurling, parting, boring to a spec, horizontal mill operation to slit on center (missed it by a little), grinding curved form tools, etc. Would be a heck of a semester final project.
I made a 45 ACP taper crimp similar to that. Mine does a pretty good job. I wanted a steeper angle than the Hornady I was using. What I was after was a good tight crimp but only at the very end of the case.
Interesting plan. If this is made from drilled out all thread rod, then perhaps the slot cut on the end was to allow a tool to thread the die in or out of the press. That would be a lot of work to form the lock nut, so would have thought it easier to drill and tap an existing fitting. Perhaps something out of a "junk" drawer of left over parts taken off old appliances.
Especially when 7/8 jam nuts are available in most hardware stores for pennies. I suppose the die could also have been made from the threaded portion of a long 7/8 bolt as well. This does perfectly demonstrate that good tools don't need to come from a major manufacturer.
I don't believe anyone has yet mentioned a need for the thread at both ends, I can only think of one, there may be others, but it may be a die made for the Lyman E-Z loader. Other ideas for the double thread???
I made my taper crimp with a 15° angle. Gives a good, tight taper crimp but only over a very short section of the bullet. I don’t want to squeeze down a lot of bearing surface. I also don’t want to damage the shoulder above the case mouth with the taper.
"when 7/8 jam nuts are available at most hardware stores...."
Hmmm. The standard thread pitch for 7/8 is 9 TPI. I have yet to find a single
bolt or nut in UNF 7/8-14 in any hardware store around here, and I have looked
at many of them. Not saying they don't exist, but certainly not common around
here. Not all have ANY 7/8 bolts/nuts and all I have checked in the last few
months that have 7/8 bolts/nuts are all 9 TPI.
Trivia note: the 7/8 - 14 was the original thread size for Budd wheels studs used on trucks from the 1920's to the metric conversions. Pacific Reloading Tools standardize on that size of die in 1931 because they were good material and they could buy replacement studs by the barrel load cheap. Don't know what they use now, but the last fire engine I worked on had made in China axels and metric nuts.
Budd inner and outer nuts for 22.5 and 24" wheels on heavy trucks have been 1-1/8" x 16 with 3/4" studs for decades. I have a 1957 2.5-ton Chevy with the 7/8" budd and 5/8 studs, probably the original Budd dimensions.