Yes it is stock. The standard trans for the Special and Century was the 3-speed the optional trans was the Dynaflow. The Standard Catalog of Buick 1903-2004 (3rd ed.) lists the Daynflow as a $204 option on the Special and Century. About 2% of Buicks came with the manual Synchromesh transmission. In '56 the transmission was the strong 6-bolt top cover trans. It is a strong trans, about the same as the LaSalle's and much better than the 5-bolt top cover trans. The numbers you are showing appear to be the numbers from the tailshaft housing. For '56 my chassis parts book lists the tailshaft housing (aka transmission rear bearing retainer group 4.317) number as 1168512, but the books often leaves out additional numbers so 1168513 could also have been used in '56 as well. There should be some more numbers on the trans main case. Their location varies somewhat by year, but I think on a '56 they are on the passenger side and probably upside down if looking from the bottom.
Check here from more info on the numbers:
http://www.teambuick.com/reference/years/56.shtml
Post a picture of the cowl tag and vin tag (drivers door jamb) if you have them.
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