The block limit signals to which bluebelly is referring, were originally mounted on the old semaphore block signal masts that were usually located in front of, or somewhere close to, a depot block office.
The original block signals were given identifying "call letters" usually coming from the name of the depot in front of which it was located (Manorville was "MR", coming from "M"ano"R"ville. Calverton was "AH", coming from it's previous name of B"A"iting "H"ollow.)
When the block offices were closed, either part time or permanently, these semaphore block signals were replaced with unattended block signals, which were the lamps mounted to the signal mast.
Years later, when all that was required was a block limit signal, either the masts were cut down to the short style and capped, or newer, short-style masts were installed. Over the years, even more modern forms of this signal have been installed.
Here's an example of Mattituck ("K") with the tall mast, sporting semaphore blades and unattended block signal lamps:
And Mattituck 9 years later with the shorter mast, displaying the block limit signal (the shelter-shed at the right is preparing to take over as the replacement depot):
Dave Keller