I may be wrong but I think Bicentennial Silver Sets were proof and came in rigid holders. That said, I've also seen tarnish and spots in both proof and mint sets. In humid environments moisture is the coin's enemy and seems to sneak in even mint holders. Not sure about the missing reed.
If the missing reed is missing at 180 deg. it is from collar. The silver spots by the most recent studies could come from cleaning residues, and in contact with the caps material, with time accelerate formation of carbon spots. It is not an definitive conclusion and studies continue., it is an preliminary supposition and Canada, US and others mints country work on the issue.
Parnelli917 - Bicentennial Silver sets were also available in Uncirculated sets. They were packaged in cellophane and in red envelopes similar to the blue IKE envelopes. The set had a cardboard frame that you could pull self-adhesive strip cover off of to frame the cello.
Yes, if the collar is not tide correctly them an laps between the two parts of the collar will have a gap. Every side of the collar cover half of the coins. The max they can be will be equivalent wit one rim reed. In your case of the coin if one side has a missing side, at 180 degrees must miss also because the collar was open in the roll.
According by the studies this can happened and will be readjust after 2 min of striking in the modern strike. I have no data for 1976 strikes so I can not anticipate the frequency. If it is this is sure it is not a frequent strike error.
I can not diagnose only by only one side. Also in old time was see that due of some thicker planchet, the collar open upon the force of the strike.
Many thing that the Mint change the chain of production just for the faster production. This it is a half reason, the main it is the Mint want the most durable and less error or varieties to occur.