The dtb file is used in different locations now. Some of its content has moved to the boot loader itself, so this file named in extlinux.conf or found in “/boot” is not a complete reflection of what your system used. See the above posts about using flash instead of extlinux.conf.
You’ll see in the posts earlier in this thread what basically is an evolution of getting both the TX1 and TX2 to use the same R28.1 L4T version (the driver packages differ, but the kernel and rootfs are now the same). Because of this change device tree ended up moving to within the boot support partitions.
To see what your system actually used instead of a file you think was used always look at “/proc” (this would have been true prior to the device tree changes of R28.1, but back then we knew the extlinux.conf named dtb file was used so it wasn’t really necessary). You can reverse compile the exact idea of what the kernel thinks the device tree is via:
dtc -I fs -O dts -o tree_from_proc.dts /proc/device-tree