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Thanks again fellas! Based on the information you have provided, I looked up the GT350 manifolds and found the following: Sounds like I may have a very early one based on the area I bolded below. Thanks again!

Intake Manifold
Variations in the "small COBRA lettered" intake for the 1965 GT350 street cars had to do with the location of the firing order embossed onto the intake as well as the S1MS-9424-A part number.

The very early GT350 Mustangs, up to about serial number ??, did not have a Shelby part number on the Cobra intakes. These were also the manifolds used for the 289 Cobra street cars as the optional "hi-rise intake" while the cars were at the Venice factory.

Most of the 1965 GT350 production intake manifolds had the Shelby American part number on the top of the rear runner behind the carburetor. The firing order was usually located on the far back, top, flat pad of the intake. Areas to look at if an intake is an original or a reproduction is the opening where the carburetor is mounted. The original intakes have four circular holes whereas the reproductions have oval slots openings. The 1965 and early 1966 intakes also have a stamping on the underside, near the front. It is on a round, about 1 inch in diameter, plug like casting. It reads OECO , which in short, stands for Offenhauser Equipment Company, the manufacturer of the manifold. The intakes were machined by Dearborn Steel Tubing. My intake also has the letters J N as well as A WL, which may have been stamped years later as a means of keeping track of it. The rear of the small lettered intake has another one of these 1" raised round plug castings, mine has a strange 303 with a very, very small 6 next to that.

Just for comparison purposes, the competition GT350 intake had no PCV hole or electric water temperature sending unit holes drill and tapped. PCV valves were not used on the race cars. Water temperature was monitored by a mechanical gauge which had its sending unit installed in the hole usually used by the heater hose.