This is the first time I’ve ever heard of this pencil. Apparently it has a digital clock? Tip looks to have a siding sleeve. I don’t know what lead diameter it could use, perhaps 0.5mm.
Here’s a current listing.
Below is a picture from Google of the Windom in its case.
Looks like a Cross pen to me. The clip really screams Cross.
The more I look at this the less I think it is a Cross pencil and the more I think it is not a Pentel pencil either. Maybe an early to mid 80’s trash pencil.
Stop looking at it so much. 
The design looks to be a body knock pencil from Kokuyo’s later Mistral line. This design was also licensed to rOtring in the late 90s-early 2000s. Addition of a digital clock seems to be targeted at students taking their university entrance exams. Seriously doubt it’s a Pentel.
Photo 7 on the listing has paperwork, and nowhere on there do I see Pentel (ぺんてる).