I think that the famous “mountain” gorillas “Ngagi” and “Mbongo” at San Diego Zoo in the 1930s were really Eastern lowland gorillas.
They were definately Eastern Lowland and are listed as such in the 2010 International Studbook.
At the end of this studbook is a historical listing of Mountain/Eastern Gorillas held in Zoos. However some of these listings are either innaccurate, or debatable (e.g. some of the earlier Antwerp gorillas; Rundi & Tanga at London were definately Eastern Lowland, not Mountain, though Reuben was definately a Mountain. Not sure about Meng)
'Sumaili' at Bronx Zoo was an Eastern Lowland, not Mountain.
The much earlier 'Miss Congo' at Bronx was a true Mountain Gorilla though seems not listed at all.
Toni at Hanover was definately Eastern Lowland, as were the 1950/60's imports to Antwerp.
'Mkubwa' at Oklahoma/Houston was definately Eastern Lowland, however the female 'Josepha' (from Tel Aviv) was an ordinary
Western Lowland Gorilla.
Despite the longer list, I know of only four undisputed Mountain(beringei) Gorillas which have been in Zoos;
f.Miss Congo. (1920's) New York Bronx(not listed)
m.Reuben. London Zoo.
f.f. CoCo & Pucker- Cologne. Germany.
(Some of the others listed may have been too but will always remain debatable now.)
'Cross River' Gorilla('deihli') only the one female at Limbe is listed. Any others entering captivity in the past wouldn't be identified as such as its a recent classification- but its quite possible they did, given the lack of clear differentation features from other Western Lowlands.