They had a loop in the top of the adapter and just basically set it over the existing Std Gage knuckle. It was a casting only and had no moving parts. See photos from Lynn.
In regards to what rules, the only details I am aware of is that Std gage cars rarely went south of Three Springs, PA due to tunnel clearances at Sideling hill and Rocky Ridge tunnels. However, somewhere I read that a Barber Asphalt Company tank car was as far south as Robertsdale. However, pehaps some one has an old EBT Rule book that covers handling of Standard Gage cars on narrow gage track?
There was a scrap shipper in Three Springs. I have seen a bizarre photo of a 65' PRR scrap gon on the head end of a Northbound EBT train in the 1950s. Photo was taken on the long fill as the train was crossing Runk Road. The size difference between a 65' Standard Gage gon and the little EBT hoppers and Mike were amazing. I believe that EBT handled Std Guage cars until the end.
TAH