Looks like VW tried the old SD455 EGR scam...

[DaJudge]

I was thinking the same thing Steve. I am working on a 73 TA right now that was built after the change and had to round up the two thermostatic vacuum switches, spark delay switch in the head, spark control solenoid, the temp sensor in the head and the trans mounted TCS solenoid. I was looking at an original service manual with the earlier system schematics and that timer was a pretty ingenious way to beat the system lol. One thing I have noticed is that a lot of people are painting the engines built after 3/15/1973 the wrong pre 3/15/1973 color.

What has been a plus is that the emissions components for the base TA 455 is the exact same as the SD455 making the parts a little easier to find, still pricey but obtainable.