MZVPV256HDGL-00000 -> new SM951 NMVe version
MZHPV256HDGL-00000 -> common SM951 AHCI version
MZHPV256HDGM-00000 -> GL and GM refer to NAND package density. Afaik, there is no 256GB version with "GL" NAND. I guess somebody took the PN of the 128GB SM951 (MZHPV128HDGM-00000) and just changed the 128 to 256.
MZHPV256HDGL-000L1 -> Last two refer to specific customer versions. L1 for lenovo makes sense.