Case question

[Landersknives]

On the 54 pattern trapper. I know the early xx and USA years and some early 70s the pattern number is stamped on the back of the tang on the clip blade. In the mid/later 70s they started stamping the pattern number on the Spey blade tang. So you would have the Case stamp with the dots on the master blade and the pattern number ie. 6254/3254/5254 stamped on the Spey or secondary blade. Does anyone know when this change happened? When I look at a trapper on Evil Bay I always look and see where the pattern is stamped to help me determine if the knife is as early as the seller says because we all know sometimes you can’t see any dots. So again when did Case stop pattern stamping the back of the master/main blade tang on the 54 pattern? Just trying to gain some knowledge. Thanks for your answers and help.