Peter, I find it interesting to look into old techniques and to try to reproduce an artifact, but I think it is important to do it the same way it was originally done, or as we assume it was. Almost every handmade item tells a story of its manufacture, So, I am a bit surprised that you as an archaeologist are not taking that way. It would not have been a lot of work to forge a TSUBA blank from scrap iron and to make some cut-outs with small chisels. This was very probably the method these early TSUBA were made. Keep us informed about the progress of your work, and we will see if it comes close to a TOSHO TSUBA. TAMAHAGANE can have differing content of carbon, as far as I know, and the usable parts (there are pieces of pig iron that are not usable) range from very low C content up to something like 1.3 %.