Can't flash stock recovery 4.20.531.4

I downloaded the stock recovery for 5.0.1 from here http://forum.xda-developers.com/show...32&postcount=1
When I go into fastboot and try to flash I get an error:

$ fastboot flash recovery Tmo-M8-4.20.531.4-recovery.img
target reported max download size of 1826152448 bytes
sending 'recovery' (16046 KB)...
OKAY [ 1.753s]
writing 'recovery'...
FAILED (remote: image error! (BootMagic check fail))

This is with bootloader unlocked, if I try with bootloader locked I get:
$ fastboot flash recovery Tmo-M8-4.20.531.4-recovery.img
target reported max download size of 1826152448 bytes
sending 'recovery' (16046 KB)...
OKAY [ 1.757s]
writing 'recovery'...
FAILED (remote: not allowed)
finished. total time: 1.763s

I notice theres a 1mb difference between twrp-2.8.7.0-m8.img (17.4mb) and Tmo-M8-4.20.531.4-recovery.img (16.4mb)
How am I supposed to get the stock recovery back without erasing anything else?

Device is S-OFF