Medic --
This could well be an original, untouched 1953 SA rifle, the serial number is post WW2 and in the Sep-Oct 53 period based on the information below.
There's no exact details of post WW2 production dates (which started in 1949), but the stock marking of the DOD Eagle started circa Sep-Oct 1953. OK for the stock.
Barrel dates usually match receiver dates (within a couple of months), so your barrel matches nicely. In addition rifles starting approximately in the 4,320,000 - 4,350,000 range had the little Eagle stamped on the barrel on the barrel. Your's is only a few 1000 off that mark and barrel and receiver production wasn't paired. Next receiver got the next barrel out of the bin so there's always considerable overlap. This further suggests Sep-Oct 53 time frame. First in first out is the production cycle, but folks get busy.
The C on the barrel is probably an inspector's mark, the P is a proof mark. Two Ps could just mean the guy with the hammer hit it twice!
Major part numbers should be:
Bolt 6528287 SA
Oprod 653582 SA or a subcontractor for SA
Trigger housing 6528290 SA
Common theme for 1950s parts is no drawing letter prefix as in earlier versions. Too many to list in an Email and there are nice books with all the details.
The 87 is a "rack number." Painting these was expressly forbidden by the Army, but other services weren't so picky.
-- Chuck