According to my source of all things Betelle:
“Betelle’s offices were in the Chamber of Commerce building Newark. Maybe they have leasing records that show when he occupied the offices, and who took over the office. I can’t imagine that his company simply dwindled to the point he was the last employee. Possibly there were some second hands that took over and reincorporated under a new name. That said, Academic Gothic was truly out of fashion by 1954! He might have just wrapped it up around WWII and called it a day.
“Also, since Betelle lives in Short Hills, I wondered what their library would have. Sometimes people just dump things on the local library. I’d be curious what Betelle’s house looks like. Also, there would have been a will. There was an estate that had to be discharged. Betelle designed the hall of records, it’d be nice if we found the details of his estate there!”
I’ve had this urge to find the exact office where his firm was, and convince the current tenant to let me dig around. You never know what you might find in some forgotten storage room…