The following explanation comes to me from a retired U.S. Army supply officer:
To understand this is to understand the Big Army's post Vietnam inventory mentality.
When the M261 was first introduced the Army serialized almost everything including
some equipment and accessories that under normal wear and tear would be considered
perishable.
Eventually they realized that the M261 could not be economically repaired, but it was still
accountable for at inventory time. Units that had "surplussed" these accessories now had
to fill out extra disposition paper work. Serialization was then looked at as being a bad idea
for perishable/semi-perishable items.
With the regulation rewritten, the manufacturer was now free to drop the serial numbers.
Is he right? I don't know, but it sounds right.
Ted