You don't have a photo of the IP showing where this bolt is, do you? I've played the O-ring shuffle before and thought I replaced everything there is with Viton, but I don't recall changing this one. I don't have any leaks, air fuel or otherwise, but I'd like to know where it is and perhaps change it to viton preemptively.
I found this thread that shows the bolt itself: banjo bolt with o-ring from injector pump, OM 606.91

But I don't want to go randomly unscrewing bolts off of the IP in hopes I got the right one.

Edit: nevermind, I think I found it. Number 125 in this diagram? http://detali.ru/cat/oem_mb2.asp?TP=1&F=210025&M=606.962&GA=722.608&CT=M&cat=19T&SID=07&SGR=120&SGN=04

Edit2: What are the symptoms or consequences of this "internal" O ring going bad, if not an external fuel leak?

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'98 E300 turbodiesel

Yes, #125 is the banjo Bolt and it is sealed on both sides by a crush washer (#128). The banjo bolt is actually not a banjo bolt but rather is a pressure-relief valve and I would assume the o-ring, if leaking, would mean the pressure inside the IP would be lower since fuel would leak past the relief valve into the return line, but would not leak out of the system. If fuel were leaking out at this point it would be due to a faulty crush washer or that the bolt were loose but not due to the failure of any rubber parts.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Just to update - I had the car serviced recently and the leak was from one of the fuel filters. Not sure which one but after replacing both filters no more leaks. The dealer said all of the lines are fine......

Barry