S, Inc. M1As are all chambered to SAAMI .308 Winchester specifications, the question is moot. Since only S, Inc. manufactures the "M1A," there you go.
7.62-MM (7.62x51 "NATO") and .308 Winchester cartridges have the same physical dimensions. The oft-mentioned pressure difference isn't, as CUP is not piezo PSI; the minor difference that exists is well within the error band for rifles marked either way.
USGI 7.62-MM chambers are "longer" in headspace than .308 Win SAAMI, because M14s and M60s are machine guns whose chambers need the extra space to accomodate fouling & etc. that builds up quite rapidly during full-auto use.
Some foreign arms marked "7.62" are within SAAMI .308 chamber headspace dimenstions, such as my DCRA Enfield. Others have the "long" chamber, such as my Ishapore 2A1. If you have a "long" chamber such as the 2A1 or my M14, use only military ammunition with its tougher, heavier case. However, unless you have a USGI "Navy" 7.62-MM Garand or a USGI M14, you won't be running into the "long" chamber very often. Check headspace.
The 10/22 Complete Owner's & Assembly Guide
The NEW AR-15 Complete Owner's Guide & Assy Guide
The M1911 Complete Owner's Guide & Assy Guide
The M14 Complete Assy Guide & M1 Complete Assy Guide