Did some SP 1s have selector stops? I have never noticed this before. The M-16 in the linked advertisement is marked SP1 and appears to have selector stops. Is it a conversion to select fire? The ad does not say. Did Colt sell select fire SP1s? Did someone add the stops somehow? How would this be done to have them hold up to use?

https://machineguncentral.com/ViewDetails.aspx?p=307--b9f06b97-9ea6-4dd2-ae80-3e3cbaa901d0

Some SP1s were converted to select fire and registered as MGs prior to the 86 ban. These are fully transferable. It looks like the "stops" as you call them we're engraved later or at time of conversion.

I can tell you that the only thing that is SP1 on this carbine is the lower receiver. It has been modified to be a select-fire lower, thus making it a registered machinegun lower. Whomever built this, used that lower since it was produced prior to 1986. This is NOT a factory Colt carbine by any means.

Upon further research of this particular firearm it appears that in the 1989-1990 timeframe it was included in a seizure of a bunch of firearms from Billistics Incorporated.  Obviously it was the SP1 lower receiver with either the original, a different, or possibly no upper receiver since the upper now on it is a much later flat top with a M4 profile barrel.  Ballistics Incorporated most likely did the full auto conversion since that is one of the things they were known for.

Link to documents below containing Colt AR 15 serial number SP203634 (exhibit no. 4 on the list):
http://www.titleii.com/bardwell/us_v_billistics.motion1.txt

Interestingly an Uzi with a Billistics Inc. full auto conversion that was also on the list and part of the seizure sold at Rock Island Auction on December 2, 2017.

He's referring to the raised tabs the selector stops against. What is a good way to add those?

I've never seen a SP1 with them either.

The link to the seizure made me look into Billistics Inc.  Apparently, the guy behind it did conversions, and was known as a genius at HK conversions, but he also did others. I think when he died (and this I'm not sure about) the ATF seized all the guns in his place that he was working on.  Apparently most of them were already registered which is why they got returned.  I'm guessing that Billistics Inc did the conversion and added the stops (by welding?) and engraving and then re-annodized the receiver.  It looks black, not grey.    Anyway, here's a link for some more reading about Billistics Inc for those interested:
https://www.hkpro.com/forum/hk-nfa-talk/125111-help-history-billistic-inc.html

I thought the sp's had the word FIRE not SEMI on them also.

It was my thought as well that they (most likely Billistics Inc.) added the selector stops by welding and machining along with redoing the selector markings at the time of the full auto conversion.

It may be just the way the picture is taken but it appears that the auto sear pin hole is a little bit more rearward then normal on a factory manufactured full auto M16.  Again it may be just the way the picture was taken.  With that said if I were selling a firearm that I was looking to get $20K for I would have much nicer and clearer photographs included in the listing.