@Sumner - First, the object in question is a backup light. Here's a photo of an HO brass model that shows it very well...
Photo (1) - N&W Mallet 2-8-8-2 Y-3 Tender Light in brass:
Photo (2) - N&W Mallet 2-8-8-2 Y-6 Tender Light painted:
Using these photos as a reference, it's pretty plain that this is a backup light.
HOWEVER, in the one photo I found of a UP-ized N&W Y-3 2-8-8-2 Mallet, this backup light isn't visible on the engineer's side of the engine.
Photo (3) - UP-ized ex N&W Y-3 2-8-8-2 Mallet showing that the off-center backup light isn't there in this photo:
Maybe the light just isn't visible, but it's still there??? Hmmm...however, when I found a photo of an HO model of the N&W engine in nearly the same pose, the light is clearly visible.
Photo (4) - HO N&W 2-8-8-2 Mallet with visible tender backup light:
Since the UP purchase five N&W Y-3 2-8-8-2's, did all of them have this type of backup light? I found a brass model of a N&W Y-3 with a different tender backup light.
Photo (5) - HO N&W 2-8-8-2 Y-3 Mallet with low, recessed tender backup light:
Sooo, the one prototype photo I have shows no top-mounted tender backup light on the UP-ized Y-3. Hmmm...does this mean that UP removed the light when the engine was UP-ized, or does it have the recessed tender backup light???
In a possible answer to this question, I found another photo of an HO model of this engine after being UP-ized. If this model is correct, I am going to vote that UP removed the light and the Y-3's tender wasn't re-equipped with a UP-style backup light. But, this is only a semi-educated guess because another possibility might be that different tenders for these engines had different backup lights...some recessed and some top-mounted. This might be the case because the original engine numbers for the five purchased engines are not consecutive. But, who knows???...since there are models of these engines after being UP-ized with both recessed and no backup light.
Photo (6) - HO UP-ized Ex N&W Y-3 Tender w/o a Backup Light:
To find out for sure, I would say that a photo of the rear of the tender on one of the UP-ized Y-3's, OR a photo of the rear of one of these tenders attached to a UP Rotary Snowplow would answer the question.
I checked on the tender behind the rotary snowplow located at the Ogden Union Station and it is definitely a UP tender taken from a UP TTT, so that isn't a help.
Another possibility would be to look for photos of the particular N&W engines that were sold to UP. Their N&W numbers were: 2013, 2020, 2025, 2030 and 2041. They became UP numbers 3670-3674.
Good luck!
Cheerio!
Bob Gilmore