Sure, the part numbers and date codes are stamped in the corners of the trumpets.
Horn date code format: ymw
y = year
m = month
w = week
For example, a horn dated 8A2 = 1968/January/Week 2, so the horn was assembled the second week of January, 1968.
All GM production line horns have spot welded brackets. Horns with large nuts holding the brackets on are replacements. There are no NOS horns. Production horns have the welded brackets and replacements do not.
Paul