I know I talk about 70s Pennsylvania traffic signal installations a lot, but the oldest doghouse signals I've seen were in greater Philadelphia in Pennsylvania, these in the following picture are old 12" Eagle flatbacks, the earliest I've seen of a doghouse, the signals right here are an example, this right here is probably one of the oldest doghouse traffic signals in PA, according to an old PennDOT plan, these were installed in 1973, being at least 40 years old at the time of this post:
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi29.photobucket.com%2Falbums%2Fc266%2FMacsignals%2FDelaware%2520Lehigh%2520and%2520Skuykill%2520River%2520Valley%2520Signals%2FLevittown%2F100_1968.jpg&hash=6b0d786b4246eb14a3f36961765ee1078418b9a9)
Oldest ones I've seen were on NY 104 in Rochester, NY which had 8 in. balls and 12 in. arrows. Really strange setup. NY 104 was completely rebuilt in that area about 12 years ago so those signals are long gone.
I never saw an all 8 in. doghouse in person but have seen them on YouTube.
Oldest doghouse signal I have seen is on Staten Island in New York City. 12" Eagle Alusig. Located at Richmond Av. and Arthur Kill Rd.
There are two present at the intersection. They date back to the early 1980s, when the location was widened by the DOT. Doghouse signals are rare in New York City, and these are the only two on the island.
https://www.google.com/maps/@40.5610472,-74.1695947,3a,37.5y,283.28h,89.64t/data=!3m7!1e1!3m5!1s4UivRIJPUO55V6ZvawZGYQ!2e0!5s20121001T000000!7i13312!8i6656!6m1!1e1
Also note the intersection itself resembles a NYSDOT configuration, because of the span wire in use. The original controller is not a standard NYCDOT mechanical unit, but it is an early computerized unit.
Oldest I've seen is a Marbelite doghouse in Tuscaloosa, AL. I'll post a picture later.