Service Manual-2024 shows for GPWT:
DLTX-5 and DLTX-15 for ser# 402040 to 404809.
DLTX-17 for ser# 404810 to end of production.
For ser# 400000 to 402039 I'd go with what Sleufoot said.
As far as trying to locate one the GP also used the DLTX-5 and DLTX-15 and the BR/BO used the DLTX-15.