Putter heads look interesting. The S25C carbon steel is the Japanese designation for 1025C, but there can be some very minor variation within the specs. 1025C is a soft carbon steel that I have always liked and is probably used in this case for manufacturing because the head is milled. You don't find too many carbon steel putter heads available today although at the high end I see a trend in some circles to moving back to carbon steel.
This quote was posted a number of years ago on the GolfWRX site by a "Fairway Fred" which is helpful in understanding some of the metals used in putters and golf clubs.
"1020 is a true carbon steel while 8620 is a steel alloy that is not carbon steel but shares some properties (like rusting) of real carbon steel. "8620 Carbon Steel" does not really exist and is a marketing term coined by the golf industry. Any steel with 4 numbers that starts with 10 is a true carbon steel. For example 1018, 1020, 1025, 1030 are all used in forgings and cannot be cast. Clubs that are made from 8620 steel are typically cast (like our Scratch 8620 wedge or a Vokey wedge) or "form forged" (like some Cleveland wedges or any club made from 8620 steel that says its Forged). I do not know of any golf clubs made from 8620 that are truly forged like a 10 series true carbon steel. The second 2 numbers are the carbon content of the steel and the lower that number the softer the steel. So 1020 and 8620 have similar carbon content but otherwise are very different and 1020 carbon steel is softer than 1025 or 1030 etc. "
Some metals are easier to mill, forge, weld, etc., than others and manufactures will usually pick the metal that works best for their design and manufacturing process. This often why a 303SS is used by some and 304SS is used by others.
I was interested in the mallet head until I saw that it had an aluminum insert. I know some golfers prefer the softer insert, but I am just the opposite. I was not really that aware of this until a number years ago as I looked at the putters I was planning to sell at the time and those I planned to keep. I had tried a number of insert putters, but when I considered why I stopped using each putter which I had purchased for play, it was because I never had good distance control with it. Since then I have never purchased a putter with an insert for personal play. I know this is a matter of personal preference and ymmv.
If you are looking to put together your own putter this coming year, these two heads are really premium quality. I will be giving the blade putter head a serious look as I like the weight, design and material. I would have definitely been buying the mallet, until I noticed it had the insert.