R.Marshal,
I agree it looks like 9.3x57( if you verify your caliper measured groove diameter is too large for 9x57) to me also. The 12.05mm head diameter will not fit either of the 9.3x53Rs. The 55.08mm case length is too short, but that is easily justified. There is a gap between the rear of the barrel and the bolt face, which causes the "blob" on the end of the chamber cast. This gap varies rifle to rifle, but is generally around .200", which would explain the case length.

Mike