Any use of Huck bolts on 80# Dudley rail - even if back in the 1970s - is surprising to me, for a few reasons.
First, 80# rail has very little girder strength compared to the rail Amtrak relaid on the Post Road Branch. With 80#D there would be, I believe, a high risk for broken rail outside the joint bars, even if the joints were effectively "frozen", especially in case of any irregular surface, profile, or deteriorated ties under or adjacent to the joints.
Second, it seems likely there would have been some degree of wear in the 80# joint bars that were re-used, making the "frozen joint" impossible to maintain, even under light traffic. In such a case, the different facts could clearly warrant replacement of the Huck fasteners with conventional track bolts.
I have not seen or ridden over the Post Road branch since the early 1990s, but can say the Huck fasteners were still there as of that time, IIRC.