[alhs06]

I think your corect WGJ, below is a excerpt from Mustang 428 Cobra Jet Registry 1968 - 1970 Ford Mustang 428 Cobra Jet Registry!
found @ this link: http://www.428cobrajet.org/id-block.html
But aren't these the same engines used in the Shelby? That's what I understand, that they used the same cobra engines?

If you have access to the rear of the block, the most definitive method for verifying that you are looking at a reinforced 428 block (as used in most 428PI, 428CJ, and 428SCJ applications) is to check the rear face of the block for a scratch mark manually installed at the foundry which roughly reads "C"; see above photos. If the marking is an "A", it suggests you're looking at a standard 428 block as used in LTDs, Thunderbirds, and Mercurys. Shortages in 1968 sometimes caused the engine plant to substitute an "A" block for a "C", or vise-versa, so the presence of an "A" block is possible in a stock CJ engine. SCJs typically only received "C" blocks because the "A" block was discontinued from service toward the end of 1968. If there is no marking on the rear, it is most likely an ordinary 360/390/410 block. Since the scratch marks are year-dependent, you must verify that the block's date code shows it's of 1966 vintage or later. I've received reports of "C" blocks being cast as early as November 1967. Note also the three oil gallery plugs above the rear cam plug. All 428 Cobra Jets were originally equipped with hydraulic camshafts. If you don't see all three screw-in or press-in plugs you can bet that the block was originally equipped with a solid lifter cam and is not a CJ or SCJ block.

The following excerpt is from this link: http://www.kitcarusa.com/kb.php?aid=38

AC agreed, provided a suitable engine could be found. It was, in the form of Ford's 260 in³ HiPo (4.2 L) engine - a lightweight, thin-wall cast small-block V8 tuned for high performance. In early 1962, the first engineless AC Ace was air-freighted to Shelby's Los Angeles facility, where it was fitted with an engine and transmission in less than eight hours and taken out on test. Carroll Shelby claims the name "Cobra" came to him in a dream, and the car was given that name.
It proved to be a great performance success; the automobile had originally been designed by Shelby with the intention it would be a "Corvette-Beater" and at nearly a ton less than the Chevrolet Corvette, the lightweight car did just that. Production proved to be less easy; the AC chassis needed extensive work to take the Ford drivetrain, and at first, the cars were slow to be produced. Soon, a larger but similar Ford engine, the 289 in³ (4.7 L) V8 was fitted. Shelby Cobras began an extensive racing career with this engine, which boosted power from 165 to approximately 270 hp (123 to 201 kW) and gave it a new top speed of 140 mph (225 km/h).
1964 saw an even larger engine fitted; Ford's famed 427 in³ racing engine (7.0 L) developing 485 bhp (362 kW) and attaining a top speed of 163 mph (262 km/h). Even more extensive rework of the AC design was needed to accomplish this; the big-block FE-series engine was much too large to fit in the standard engine bay. The front tires were moved outward, with extended wheel-arches to cover them, and a new coil-spring suspension was designed.
Some Shelby Cobras were fitted with Ford's 428 in³ (7.0 L) engine, a less powerful engine despite the nominal increase in displacement, intended for road use rather than racing.

But still, my original question stand's..."Does anybody know what the XX scratch marks @ the rear of the block & the cast 17 mean, or the "S" stamp @ the oil filter area?