I am not saying they did. Just that Metric 753 used that size seatpost in some instances. I think the Trek 170 came out in 84 so it could very will have been Imperial 753. Below are notes by Mike Mullett lifted from another forum:

As written by Mike Mullett - :

1) 753 is Metric-diameter tubing (28.0 mm O.D. for the seat and down tubes, 26.0 mm O.D. for the top tube), and was introduced in about 1975. Seat posts for 753 frames are either 27.0 (for small frames) or 26.8 mm (for large frames).

2) 753R and 753T are Imperial (English/Inch) diameter tubing (28.6 mm O.D. for the seat and down tubes, 25.4 mm O.D. for the top tube), and began production in 1983. Seat posts for 753R and 753T are either 27.2 or 27.4 mm.

Of particular interest is the thinner Imperial tubing which takes the 27.4mm seat pin (pillar). Not a lot of people are aware of this. All these tube sets have a part number starting 80*, but Reynolds seem to have lost these specs.

Sorry for the hijack.