Porsche utilized Weber IDTP on more than one model of car, with subtle distinctions.

The 914-6 part number adds a "1" after IDTP, whereas the 911T does not.
914-6 70-76
  • 40 IDTP 13 C1 - 901.108.115.00
  • 40 IDTP 13 C - 901.108.126.01

911T 69-
  • 40 IDTP 3 C1 (left 911) - 901.108.118.01
  • 40 IDTP 3 C (right 911) - 901.108.118.01

There is very little difference, per Paul Abbot from Performance Oriented:
QUOTE
The Webers themselves look identical from IDTP to IDTP1, the only real difference is the location of the main fuel jet installation. The main fuel jet location for the middle barrel is towards the front of the car so for 914/6 the location of the main fuel jet is relocated from that of the 911, nothing more that that and a part number difference.