Free (small) partition for storing serial number?

do you mean this?

root@swi-mdm9x28-wp:~# more /proc/mtd
dev: size erasesize name
mtd0: 00280000 00040000 “sbl”
mtd1: 00280000 00040000 “mibib”
mtd2: 00680000 00040000 “backup”
mtd3: 00100000 00040000 “security”
mtd4: 00200000 00040000 “persist”
mtd5: 01180000 00040000 “efs2”
mtd6: 05040000 00040000 “swifota”
mtd7: 00100000 00040000 “tz”
mtd8: 00080000 00040000 “rpm”
mtd9: 02080000 00040000 “modem”
mtd10: 00100000 00040000 “aboot”
mtd11: 00f00000 00040000 “boot”
mtd12: 01e00000 00040000 “system”
mtd13: 008c0000 00040000 “lefwkro”
mtd14: 01900000 00040000 “swirw”
mtd15: 08300000 00040000 “userapp”
mtd16: 03680000 00040000 “reserved”
mtd17: 02180000 00040000 “slot_2”
mtd18: 02180000 00040000 “slot_1”
mtd19: 02180000 00040000 “slot_0”