Counters for Abercrombie’s Brigade of the 1st Division / IV Corps. For the obscurity of the battle, it’s strange how many units made their reputation at the Battle of Fair Oaks / Seven Pines. This Brigade had two such: The 61st PA suffered around 55% casualties (92 dead including every one of their field officers). But they fought fiercely: When their ammo ran out, they fell back, but had already been flanked and were forced to fight their way through a Rebel unit in hand-to-hand combat, which they did.
The 23rd PA was formed as a super-sized regiment (15 cos). They originally had over 1600 men and were forced to drill and march in two battalions, each as large as a standard regiment. Four of them were sent to another regiment. According to Dr. Tripler, they were one of the healthiest regiments.
Finally, three of the units went by different names at various points: The 67th New York was known best as the 1st Long Island, the 31st Pennsylvania was later the 82nd Pennsylvania (after Fair Oaks) and the 65th New York was the 1st United States Chasseurs (sort of a Zoave-light unit).