I am not sure just when the position of the HIN on the boat was formalized, but it seems to always be on the top of the transom, one side or the other.
Then (xx months/years?) later, a second HIN was also molded into an interior out-of-normal-sight location during production, like the FRP engine pan or, in our case, the top of the aft berth, way at the back.
Sorry to be of little help.
There are a lot of Ericson 27's and 29's from that period... they should be checking in when the eggnog wears off in a couple of days.
Best,
Loren