DWTim
"I just want to interject here to mention that Lyman gives these figures with a #429421 cast from linotype at 245 grains. Was linotype available to Elmer Keith?"
Yes Elmer Keith had access to linotype. Probably greater access than we have now as all printing back then was with type made from linotype. I would suggest you read some of what Elmer wrote. A good place to start would be Reloading the .44 Magnum which was published in Lyman's Reloader Handbook #42. You'll find in there that he like his 429421 to be cast of 1-16 tin-lead alloy. Not as hard as linotype or #2 alloy but probably giving the higher pressures you suggest of softer alloyed bullets. His "Keith load" was tested by HP White Labratory (they have a substantial reputation for scientific testing) and was found to give an average of just under 34,000 pounds with only 3,000 pounds variation.
"It is also loaded to a generous 1.710 C.O.L. A softer alloy would distort more and create higher pressures "sooner". I have a batch from an alloy that is more like Lyman #2, and they weigh in at 255 grains. I personally wouldn't push them that far, and anyway a 1.710" C.O.L. is unrealistic because the crimp would be insufficient with the bullet that far out."
I'm not sure where you are crimping your loads but with my .44 magnum loads using a solid crimp in the crimp groove on the Keith bullet the OAL is 1.713". The case mouth is right up the the bottom of the driving band and the crimp is as good as it gets. These loads fit quite nicely in every .44 magnum I've tried them in (Ruger BHs and RHs, S&W 29s, Hawes, Colt Anaconda's, Tauris and several others).
"Did Keith use the #429421 as the pattern appears today? I know that he was not happy with Lyman's revisions of some of his designs. If Keith was using a bullet with a wider, sharper or deeper grease groove, for example, the bullet would have to be slightly longer to be the same weight."
Well Keith designed the bullet then Lyman changed it to round grease groove instead of square. I've used both and can not tell any advantage of one over the other with modern lubes like Javelina. Keith designed his bullets with square grooves "to hold the lube better" as the older lubes were not as good as the ones today. If you would read Keith you would know that's why he liked the square grooves.
"I've never actually measured the hardness of a Laser-cast bullet, but other sources peg them at about 4-6 less than linotype on the BHN scale."
That is correct and if you'll read the original question you'll find the Laser Cast .44 bullets in question are 240 gr bullets. lighter than the Lyman 429421 cast of Lino. I've used the 240 Laser Cast bullets over 22 gr 2400 without problem other than the usual leading because of the hard wax lube. I usually wash the wax lube off and relube them with Javelina. Then they don't lead and accuracy is then better.
Larry Gibson