Looks to me like some sort of turning block. If so, then perhaps a previous owner installed it in order to keep the jib halyard in line with its corresponding masthead sheave. Not at all necessary, however.

:: Hello,
::
:: I was looking at my mast the other day and noticed this
:: thing near the masthead. It looks to me like the Jib
:: Halyard should be routed this way, but maybe it is just
:: a fair lead. I haven't dropped the mast as of yet, would
:: like to do it in the Fall.
::
:: I am contemplating setting my boat up for a spinnaker,
:: thus the scrutiny from afar.
::
:: Please see link to image:
::
:: http://www.keay3.net/masthead.jpg
::
:: Thanks,
::
:: Keay

:: S22 "Dayspring"