XPERIA XZ1 Compact Firmware Versions and Security Patch Levels Reference
XPERIA XZ1 has model number G8441 in all markets but Japan DoCoMo, where it is known under SO-02K. The internal product name is "lilac". There are three product variants:
All unlocked and carrier-branded devices, except in US/Japan
Japan DoCoMo branded devices.
Unlocked US devices.
Additionally, there are 43 G8441 customization versions and 1 SO-02K. You can see your customization version by dialing *#*#7378423#*#*, then going to 'Service info', 'Software info' and 'Customization Version' will have actual number (something like 1310-4371_R8B). XPERIA Companion checks this customization version and will query the server for updates for that specific version.
I analyzed firmware for various versions and decided to share the results. The values below are taken from /system/build.prop and /system/etc/prop.default. Build time is selected as the latest one of ro.build.date and ro.bootimage.build.date. Table is ordered by Build time.