Need help getting Developer Edition back to stock topic

My office recently purchased a used HTC One M8 Developer Edition device for testing purposes. I was under the impression that it was stock, but I was wrong. It had TWRP installed. The system alerted me that there's a system update, but when I try to install it, it just boots into TWRP. I managed to flash the stock recovery back to it, but when I try to apply the update, it just boots to the fastboot screen, and does nothing.

Here's all of the info I've got about the phone:

(bootloader) version: 0.5
(bootloader) version-bootloader: 3.16.0.0000
(bootloader) version-baseband: 1.15.2133156.UA03_2G
(bootloader) version-cpld: None
(bootloader) version-microp: None
(bootloader) version-main:
(bootloader) version-misc: PVT SHIP S-ON
(bootloader) serialno: FA43SWM03221
(bootloader) imei: 358718050231948
(bootloader) imei2: Not Support
(bootloader) meid: 00000000000000
(bootloader) product: m8_ul_ca
(bootloader) platform: hTCBmsm8974
(bootloader) modelid: 0P6B12000
(bootloader) cidnum: BS_US001
(bootloader) battery-status: good
(bootloader) battery-voltage: 0mV
(bootloader) partition-layout: Generic
(bootloader) security: on
(bootloader) build-mode: SHIP
(bootloader) boot-mode: FASTBOOT
(bootloader) commitno-bootloader: 0a41237a
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0

*** TAMPERED ***
*** UNLOCKED ***
M8_UL_CA PVT SHIP S-ON
HBOOT-3.16.0.0000
RADIO-1.15.2133156.UA03_2G
OpenDSP-v26.2.2-00538-M8947.0106
OS-
eMMC-boot 2048MB
Mar 3, 2014,20:18:02.0

I'm adb/fastboot savvy as a Nexus user, but the HTC world is unfamiliar to me. Any help would be greatly appreciated! I've been looking up various articles and posts for the last 3 hours trying to figure this out.