Study of an original by Sei Yamagata in Close Up Gendai Soukangou, circa 1999.
Sei Yamagata doesn’t seem to have been a particularly prolific ero-artist, but he had a pleasingly chunky style with really nice use of spot blacks. I wasn’t able to find out much about his career otherwise; to look at his art, I would assume he was a professional, but going on the afterward page in his book Graduation, it sounds like he didn’t enjoy working on manga that much…