Working down the list of running changes that took place during 1966 production at San Jose as the title states we look at horns.
Looking through examples we find that 65 style were used/installed for a period then Ford transitioned to the 66 style. Both originally were completely painted semi-gloss black except for what appears to be a mask slipped over the wire connection blade on the back side. These were also stamped with a white date code. This, and all the various locations of the stamp are discussed in another thread.
1965 - early 1966 version Sometimes referred to as the "dot/dimple/button" style or version.
Later 1966 version Sometimes referred to as the "finger/tab" style
So looking through my examples I found the last unrestored example with the 65 style on 6R147404 and the earliest 66 style installation on an unrestored car on 6R134147. Since these cars were not built in sequential order we don't know if one car or cars were built before one another and since the change required no other parts to be changed the changeover was likely a "soft" one where there were some cars during a couple of days that could have had either one or one of each style/ the latter has been observed but of course can't be confirmed as being original.
Invite other data points from unrestored examples especially to help with this change over period. Hope this helps others in their projects and efforts
Complete list (well so far
list of 66 SJ Running Changes
http://www.concoursmustang.com/forum/index.php?PHPSESSID=d97t31057mg9p0k4qqi3pfbs87&topic=14786.msg119022#msg119022
