These are quite possibly experimental UK rounds - no 3 is a Long Range Dark Ignition Tracer dating from 1961 - but without pictures of the headstamps positive identification will be a nightmare! Do you actually have these items?
More information can be found in an earlier thread on British Experimental 7.62 Trajectory Matching Tracer Trials at
The headstamp on this series is K 61 L5A1.
Yes, all the H/S are 61 K L5A1
Yup those are one and the same!