The only source I've found for that twin L-mount safety vent on the dome in HO is Owl Mountain Models. It's in brass, so is pricey. Single L-mount vents are available, but I also like the twins since they were apparently used on cars exceeding 8,000 gals. capacity.
I asked Don Tichy if he had same in plastic (I think his parts line has good detail in that material). He said "no", but thought it would make an interesting project (maybe encouraging remark?).
I'd like to grind down the top-mount vents on a few 10,000 gal. cars I have and replace them with this design on the dome side. They'd represent early-production cars that hadn't been upgraded with top-mount vents.
Chuck Greene
St. Charles, IL