But you still have not put a START to the sight plug!
All you have done is proven (somewhat) that the DUKW has always had the sight plug (none of your photos show it). I don't own a DUKW, and this is not a DUKW site.
You have NOT shown that:
10/42 is the START of the sight plug. All you are doing is finding a matching FSN in an earlier book. OK, thats nice. Now prove there isn't an even EARLIER book. You are stopping short of the goal. You see the FSN and stop there. That is not an answer.. its a CLUE.
The FSN is a logistics trick. It would have been assigned to parts ALREADY in SUPPLY. The FSN means NOTHING to YT&C or GMC. Its ONLY a supply number.
At best we can move the date of the sight plug from early 43 to late 42.. but how do we know it wasn't put on the very first open cabs in August 42?
Back to my motors on the line analogy... prove to me that the VERY FIRST motor taken off the line for DUKWs was the start of the sight plug.
Does the TM9-802 mention the sight plug? The Tm9-801 is pretty clear about it.
I think this also needs more research:
"I'm only using the FSN prefix to determine the approximate date of the part or change to the part. If it changed on the CCKW first it would have a G-508 prefix. If it was used on the DUKW first it would have a G-501 prefix. That's all I am using the FSN number for. The rest of it does not help us determine vehicles/dates or uses."
How did the FSN committee assign the parts to vehicles? Was it simple numerical order? Why would the not even in production G501 be assigned the parts of the in production for 2 years G508? My guess is SIMPLE CONVENIENCE. The G501 was not under previous contracts and the Army would not have to pay any money to RE-WRITE or RE-RELEASE or RE-DISTRIBUTE or RE-TRAIN mechanics from the old TM10's. The DUKW is staring from a BLANK SLATE. So it would be MORE CONVENIENT to start there than to redo all the CCKW TM10s. So when the Standardized parts lists WERE released for the G508, the FSN were already done, thanks to the DUKW. And that has NOTHING to do with dates of production.
That make more sense to me.
And please show me how new parts (say the deep sump oil pan kits) were not assigned an FSN in the field modification kits, but were assigned an FSN in SNLs. None of the GMC service/technical bulletins mention FSNs (that I have.. I do not have an original, just a copy of the bulletin. And that reason is because an FSN was assigned for supply purposes.... not production.