I thought someone was a 'expert' on these, maybe DeSoto or Gedlicks. The sheaths may cross over changes in the knife stamping, blade thickness, number and type of spacers, grind of the butt. It will follow the examples of others such as 124 and 119s on sheath aging. And in fact may follow 110 sheaths. Sheaths were contracted and not changed with each knife production difference change. You may have a sheath that goes with two different looking 121's that is the same.
I know that besides the tang stamps the spacers tell a story also. I do not know if leather spacers were ever used. I have always thought the models with micarta on back of the hilt were a early version but I may be wrong. There may be an article on these in the BCCI newsletter in the past, maybe start with 89 and go forward......300