I think there was more than just the bolt handle improvement of the MRC over the M70. For example, the left lug on the MRC has a "conical" or "dovetail" cross-section to it, as shown in the photo below. The left-side raceway has a matching shape, as well. This more securely guides the MRC bolt as it travels in its raceway. Also, the bolt release on the MRC is better executed than on the M70.

[Linked Image from photos.smugmug.com]