I'm not positive, but I think this is A-16566 "Divertor Actuator Shaft Assy." mentioned in the manual, section 2-24. I can't seem to find it anywhere, but I found this:

http://www.marcospecialties.com/pinball-parts/A-18583.1

I think A-16566 may be the entire assembly shown in your picture (broken cam plus shaft).

There's also this:
http://www.marcospecialties.com/pinball-parts/A-15569

Not sure what the difference is. I just searched for diverter actuator.

Why not just take the one (A-17615) mentioned in the STTNG parts list and labeled "STTNG" in most shops?

http://www.marcospecialties.com/control/keywordsearch?SEARCH_STRING=A-17615

(A-16566 ist the complete Assy including the plunger, by the way. These were made by Williams back in the day, today most parts are only available seperatly and not as a completely assembled assy any more.)

Oops, out of stock at Marco - but available here: http://bayareaamusements.com/Merchant2/merchant.mvc?Store_Code=BAA&Screen=PROD&Product_Code=GS-STTNG17615

Sorry, missed that info. But if you can't find it anywhere else, you can always rely on Mark from PSPA :

http://www.pinball.co/search.aspx?searchTerms=A-17615

I just sold one of those!...sorry....bad timing.....

Mike I told you I can weld it. You will barely notice its welded. It will be stronger than before. And uh.....Its Free

This is an unusual part to break in this game. It should not get loaded to strain this plate. If the diverter assembly is aligned properly the repaired/welded part should be adequate.

As opposed to having a spare I'd look at the physical alignment of this to the assembly and make sure it is square/true with no binding. Once set up right this should be a non issue.

Mike O.