On the A2 lower the front pin detent is held in the fence. Since my understanding is that an A1 has no fence, where is the front pin detent?

A1 "slabside" lowers are mad by Colt, and dont have a pin detent.  They use a screw together pivot pin, or use a detent ball pushpin, where the pin has it's own detent.

Thanks for the response. Is this what the military was issued during the Vietnam war? Were they all held together with the 2 screws. For some reason I thought that the push pin was an "old" design.

Someone older and wiser than me will have to answer this.  I was referring to my slabside AR SP-1.  I am not sure what the Vietnam era M16's were...

I’m pretty certain the Vietnam era M-16’s used two pins (a take down and a pivot) for disassembly.  I don’t think the military has every used screws in place of the pivot pin.

The very first M-16’s were slab sides.  I have no idea where the pivot pin detent and spring were located (though it seems it would have to be in the side of the magazine well).

At some point, a long external rib was added to the lower receiver, directly underneath the ejection port, and apparently the detent and spring were moved there.  (If the ejection port cover is open, this rib is almost entirely covered and can’t be seen.)

Later, this rib was extended to protect the magazine catch and became what is now referred to as the magazine fence.  

I’ve seen A1’s both with just the rib and with the full magazine fence.

No.  I'm not sure about the model 01 (1st m-16 used in nam) but every one after had a detent and push pin.  The model 01 was a true slabside and from what I can tell didn't have any kind of pin/detent, but it certainly didn't have two screws.  The XM-16E1 didnt have the mag fence, but did have a "ridge" (part of the mag fence) that housed the spring and detent.  The VN era classic M-16A1 had the full fence and detent setup.  The Two screws is a Colt civilian AR-15 thing to try to prevent easy conversion to an M-16 upper (hence the different hole sizes).  No military version ever had the two screw setup.

A raised fence on an A1?  I thought raised fence was an A2 feature/timeline?

M16A1 absolutly had the raised fence.  It was one of the major changes from the XM16E1. Check out The Black Rifle. I think you are mixing up M16s with AR15s.  Only AR15A2s had a mag fence, and only the later (blue label ones)had it.  Early 85-91 A2s had the slabside.

I have seen XM16E1 marked lowers with the mags fence. After 8 years of military service, I have never seen an M16A1 without one.

I wonder why Colt would not add these better features to the same time frame AR15's?

Cost, it cost Colt nothing to not add it, and people would still buy the gun.  The only reason they added it later was because they totally eliminated the SP-1 separate production line for the lower and used M16A2 lowers that were modified (or not, depending on how you look at it) for the civillian production which actually further reduced their cost.  It's all about cost.

The VERY original AR-15 had a separate pin and no detent.  Sort of like a HK pin.  You punched it out and it was free to be lost in the grass.  The M16/M16A1 had a ridge along the top of the lower that contained the retaining detent pin.  They are all push-pin models.  Later M16/M16A1s had the mag fence which was just an "appendix" on the ridge that came down and went around the perimeter of the mag release.  Remember that there isn't a deirect linear advance of the M16 series.  It didn't go M16, XM16E1, M16A1, M16A2, etc.  Remember the USAF used M16 without using the A1 and had all the updated features on later rifles.  While that's the timeline of use in the service, many of the features carried into various different models.  There was an on going Product Improvement Program that incorporated things like trapdoor butstock, etc.  Those items appear during production and are just added to new rifles during the production run.  As older parts broke and were replaced, the new versions were also used.  So if you were to find an XM16E1 out there, it may have round handguards, since that's all the Army has in the system now.

All the military lowers are push-pin with the same design.  The mag fence is just an extension of the ridge for this.  The two screw is strictly SP-1.

Ross