I have been wondering for a long time whether the rusty master cylinder I found in the trunk could be the factory installed piece. The car in question is a 68 GTX that is mostly original. It had been abused in the past, but mostly by neglect, not by alteration. Upon sandblasting, I found the following:
Bottom; casting# 2660809 R 2
Mounting flange; E257
Nose (top) S
Nose (front face) 174 7
The scheduled production date of this car is May 24, 1968. I know original pieces are extremely rare, as they deteriorated rather quickly independent of use or mileage, but it would still be pretty neat if it was the factory one. Anyone out there know how I can tell?

I have a 68 Road Runner with a SPD of 504 (4 May 68) and the master cylinder has a date code of 130 8 which is 9 May 68. I know it is the original one and the dates from most other items from my car fall between late April and mid May. The car was delivered in early June. Going by the date you have I suppose anything is possible...could be that your Master Cylinder was at the bottom of a pile and newer ones put on top. The hood latch arm is my odd-ball, it has a date of mid January 68 and I just assumed that it was a bottom of the pile thing because the latch and catch are mid May dates...go figure. The only dates I have from late 67 are engine items but that is typical for the Hemi motors.
Hopefully you will have a few others chime in for comparison.

Jim

It could be but you really will never know for certain. The casting number is correct for a drum brake car. Problem is that the date code could be '67 or it could be '77. Bendix made these master cylinders into the 80's, after production of cars using this style stopped with the 1970 model year, Bendix continued to make them as replacement parts for a long time since they were used on so many different cars.
Even if a car was stashed away after purchase with no mileage and found today, the master cylinder might not be the one installed on the car when it was assembled. It is possible the master cylinder had been replaced by the dealer after a problem was found with the brakes during prep!

Thanks for the reply Jim - so the date code is the one stamped on the front nose? If so, it would be a really early built part for a May '68 car. What numbers do you have on the mounting flange- mine show up as a raised part of the casting between what looks like two philips head screws. Any idea on what they mean? Like your car, this may be my only oddball part, as most everything else put on the car is in the 120-127 day of '68 range.

The numbers on the nose is the date code for the manufacture of the assembly.
The numbers between the screw marks is a date code for when the sand casting was poured. They have a steel pattern that they would screw a plate with the date on into, and then form the mold with sand.
I haven't been able to decode that date code, it appears to just be a day, with no year, though I am not certain.

Mounting flange; E257 casting date 5/25/67 or '77

Nose (front face) 174 7 machining or assembly date 6/23/67 or '77

WAG can someone confirm?

Well, I guess I'll never know for sure. Are there any markings on the cover that would narrow down the date of manufacture? Mine has no markings whatsoever, but has the single bolt with the "S" clamp. Original finish (from non-rusted underside) was zinc -dichomate. Even if it could be determined that it is indeed a '67 unit, it seems that it was built way too early for my car. But then, why was it in a box with other original (fuel filter with KY stamps, front marker lights etc) parts? Oh for a crystal ball!

There is nothing on those caps that would give you any clues.
I don't know that the date can really be said to be "too early". A part could have ended up falling into a nook or cranny in the storage area and found later and installed on a new car, some samples could have been taken off the line for QC checks or testing and returned months later, who knows.

Well, either way I guess it should go back on my car, as it seems to be the correct looking part(after a resleeve and a rebuild kit). In a related matter, my brake drums are also cast with an "E" on the same type of marking as the "E257" on the master cylinder. Maybe a prefix denoting the brake system as a whole, or a coincidence, don't know. Thanks for the input guys, I appreciate it.

As others have said, the stamp on the front would have been the assembly date. I found this picture from when I rebuilt my master cylinder. I did not write down the casting date and I can check it out later today after work. I'll post it for comparison against yours.

Jim