While the CCKW and G506 look similar( fenders, axles & hubs, etc.) there are a lot of differences. Most G506 trucks were closed cab, while most CCKWs were open cab. They also had different engines, transmissions and driveline. I have only very limited knowledge regarding CCKWs.
G506 answers:
Early (Y and Z models) dataplates were located on the dash and the header panel over the windshield. N model trucks were all moved to the header panel.
Y model plates
N model plates
G506 truck serial numbers were stamped on the dataplate, but not the frame from the factory. If a truck went through an ordnance depot rebuild, it was common to stamp it on the frame.
Y and Z model truck serial numbers only indicate standard or long wheelbase (YP, ZP are standard; YQ, ZQ are long wheelbase). N model truck dataplates indicate the type of truck (G7103, G7107, G7117, etc) on the plate. The serial number itself also tells you the type ( NM, NL, NJ, etc). The G506 SNL parts manual has several pages of tables listing the different model types. I am constantly referring to my PDF version of that manual to determine correct part numbers.
Model Identification Table
Regarding the early winch extensions and bumper availability, the same applies to early and late trucks. The extensions are only available by removing them from a donor frame, as they were military specific. To my knowledge, they were not offered separately, as they were hot riveted together with the frame, and considered one part. The front bumper can be reproduced (I had one made for my G506). You just need an existing bumper to copy or a good fab drawing. I can't help you with a drawing of the early style, but can if you need an N model bumper.
Vendors that I know of in no particular order - Frank Von Rosensteil, Warren Duchesne, Peter Debella, Rex Ward, Marty Fravel, David Wildenberg, Frsnk Dr Rek, Jeepest, Jeep-Dodge-GMC, Brent Mullins, Vehicles Of Victory(very difficult to contact). I probably forgot about few. You will find that they are located on both sides of the Atlantic. For many parts, the challenge is finding them(why I use my SNL manual so much). Sometimes the only place to get a particular part is across the ocean.
PS - Ebay can also be a good source, if you know correct partnumbers (reference the SNL). Many sellers will list the partnumber on the box, but have no idea what it is for. TM-WW2.com is where I bought my PDF version of the G506 SNL.
PSS - parts needed will depending a lot on your objective. Are you restoring it, and need "correct" parts, or just want it to drive and stop? It makes a HUGE difference!