Quoted:
You are asking for it putting in the FA trigger and disconnect IMO. You are saying you bought it new and it came that way?
I'm just not buying that. Especially since you are passing this down, you need to put AR15 FCG parts in it. The ONLY parts I would be okay in using are a neutered selector, neutered disconnect and FA bolt carrier. I am not a lawyer nor do I play one on TV, but I personally think you are making a big mistake using FA parts. I simply do not understand your reasoning, and why you don't believe us when we tell you it's a mistake. Don't do it.
I think the OP's reasoning is...well...reasonable. That vintage SP1 would have come from the factory with a FA trigger and disconnector, so it's a very logical argument that it's justifiable to run it today with those same parts.
On the other hand, district attorneys and grand juries are not always logical. In the eyes of some, you wouldn't be running the
original parts, but would have actually
selected "automatic" parts to put in the rifle, knowingly accepting the risk of multi-round firing.
On the third hand, I'd think any semi-competent attorney could use the "original design" argument to defuse that charge. Not to mention that the only reasonable likelihood that this would ever be an actual issue would be if you were involved in a shooting, or an LEO watched you shooting and your rifle doubled, or there were some other fairly unusual set of circumstances.
On the Vishnuvian fourth hand, even a small chance is still
a chance.
Bottom line, OP...weigh the risk for yourself!