There is a difference between 55 and 56 links and they are not interchangeable. In 55 the receiving end or cup part of the link was facing upward. While it did have a protective boot it still collected water so was prone to washing out the lubricant and rusting. The contour was also slightly different and when under heavy load was prone to an intermittent sudden slipping in the cup resulting in a snapping sound.
In late 55 and for the 56 models the contour of the link ends was ground differently to eliminate the slip and snap sound.
https://packardinfo.com/xoops/html/downloads/STB/55T-37.pdf What makes them year specific is for 56 the cup was repositioned to be facing downward so water could not collect. That also eliminated the need for the rubber boot so it was dropped. You can see the placement of the cup differences by comparing the 55 rear assembly in the referenced tech bulletin to the 56 assembly in the front suspension layout in the parts manual.
https://packardinfo.com/xoops/html/modules/partslist/viewplate.php?cat=26&PlateNumber=31G&partslist=1955-1956 The grooves to visually determine length of the front links were added later so if your car is early production it probably will not have them and you will need to measure. If you have the grooves, 1 groove is link #1 and 4 grooves is #4 so you can count the grooves per 55T-1 to see what is there now. There was a reproduction adjustable link made for the 56 models but not for 55. The link rods or bars are ordinary steel so for 55 several have made new longer links by drilling a hole in the ends of a steel rod matching the diameter of the original link. The hardened link ends are knocked out of the original link and pressed into the holes in the new rod. I believe there is a limit before the load arm and link is positioned rather precariously so IIRC, it is not recommended to go more than approx 1/2 in longer than the longest stock link.
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