Yes that is only the cast number for the U-Joint piece itself not the number for the shaft. As mentioned the number for the drive shaft it self is supposed to be on the tube but usually long gone from rust many don't even have it that is why i wondered where you found it also being such an early number
I found the number about 6 inches in from the one end after I got all the rust off. The numbers were very faint and There was only one number that was gone, so looking it up in the parts book and because it came with the parts car, a 340 Demon with a 727, 2883690 It makes sense. The only number missing was the 6. Plus the measurement was also correct for the stated part.