These are now available separately from the full Bosch kit we sell here. This O-ring fits all Bosch VE injection pumps from 1977-2003. The distributor head is the part of the pump that the injection lines attach to, and where it mates to the IP is a common place for leaks due to a failed/flattened/frustrated O-ring. This is something that with great patience and a modicum of skill can be replaced in situ without removing the IP from the vehicle.
It's worth noting that as of 2011 all of our TDI conversions now include a rebuilt TDI inj pump with new Viton seals.
Available here: https://www.greaseworks.com/store/product_info.php?cPath=27_40&products_id=693
TDI/VW Bosch VE distributor head injection pump Viton O-rings: