It's been a while since you left this question, but if you still need help: At the start of WWI draftees from the same county were almost always assigned to the same training camp, and usually the same unit. Conscripts from Clinton County NY, where Plattsburg is located, were sent to Camp Devens in Ayers, Mass, the headquarters of the 76th division, which was largely a training and replacement division. Once trained, its men were assigned to other divisions as needed. So there's a good chance your uncle started, at least, in the 76th division and at some point was transferred to the 42nd.
Your best shot at finding out for sure is to fill out and mail a service records request to the NY State Archives. It costs just $1 and if your uncle is on record (most wwi vets are) it'll tell you, among many other things, the units he served with, and the dates thereof.