DonT

The E reversed with P in a circle denotes the Paris proof house
16.8 = 661.5 thou = 16 gauge
6.5 is 6.5 centremetres is the chamber length.
PT and PS with a star over the top is the types of powder used in the Paris proof house
PS = is for barrels proofed with type S powder introduced 30th march 1896
PT = is for barrels proofed with type T powder introduced after 1900

By magnifing to 400 I see the PT with crown over the top is the St Etienne proof house
I think these proof marks are pre 1923

I feel that the other marks are post 1965.
I have been through the proof book from cover to cover and cannot find any thing that relates to the marks listed below
The two Rs (one reversed) with the other heavy stampings
CG Paris
Monoblock
3988
S
The 3988 does not relate to any French pressure test figures in this book either PSI , KPA or Bar
\I don't know what it is

I hope this helps

sbs470