Upgrading from Lede to latest Openwrt zyxel nbg6817

Installing and Using OpenWrt
1

I need to upgrade a zyxel nbg6817 from SPI-NOR (LEDE partitioning) to SPI-NOR (OEM partitioning), this is the current proc/mtd output

cat /proc/mtd 
dev:    size   erasesize  name
mtd0: 00020000 00010000 "0:SBL1"
mtd1: 00020000 00010000 "0:MIBIB"
mtd2: 00020000 00010000 "0:SBL2"
mtd3: 00040000 00010000 "0:SBL3"
mtd4: 00010000 00010000 "0:DDRCONFIG"
mtd5: 00010000 00010000 "0:SSD"
mtd6: 00040000 00010000 "0:TZ"
mtd7: 00040000 00010000 "0:RPM"
mtd8: 00080000 00010000 "0:APPSBL"
mtd9: 00010000 00010000 "0:APPSBLENV"
mtd10: 00010000 00010000 "0:ART"
mtd11: 00010000 00010000 "0:DUAL_FLAG"
mtd12: 00210000 00010000 "0:RESERVED"
2

This does not compute, the 4 MB spi-nor is never touched by OpenWrt, it only contains bootloader and calibration data - OpenWrt only uses the 2*(4+64) MB on the GPT partitioned 4 GB eMMC.