You'll do just as well if not bette with a #4 Phllips on a 3/8th drive, but you can follow whatever path; after all, what would I know? My only contribution around here is spending almost 2 decades as an automotive mechanic. And if you think condridicting advice from people who don't even do basic mechanical work on their vehicle--let alone full blown restos--go for it. The only vested intererst I have with this issue is letting people know that they shouldn't squander their money because they have been misinformed.
This isn't to have a posi drive share the platform; instead, it's to completly discredit the posi drive as a fastener used on the car. As for it being a non common fastner, it's was a super common fastner in its era. Half of the Chevy/GMc trucks--maybe others--had the beds assembled with them.
My best guess is the additional pois slots have been wallowed into the head. Sometimes it can be hard to tell.
Here's a good example of it being impossible to tell:
On this port, some ports have casting markings, for instance, GM 12 would be a good cantidate for a casting mark. On other identical ports, some heads don't have the casting mark.
Did this head have a casting number? Many people have studied it carefully, and you cannot tell whether or not the port had a casting mark--you something like GM12. (The second picture relats more to did they wear off or removed because it's obious that the center ports aren't marked as they should be.) How would I know the answer--dayt bee like dat when I got them;)
Just like trying to determine what's up with the heads it's impossible to determine what's up with the internal area. The red flag is the way the posi driver fits the head compared to the Phillips.
Unlike the Phillips filling the slot at all 4 corners, the posi drive is wallows around in the hole, not filling it up on all 4 corners. The reason it doesn't fill it up on all 4 corners is that it isn't the correct drive for the fastener.
Perhaps overlooked on a casual glance, if one looks closely that the pictuers, it's obvious to anyone with strong mechanical aptidude.
