In the late 1950's and early-to-mid 1960's Skagit began to experiment with swinging 'Guylineless Towers', or GT's. (Later the term 'GT' was adopted as 'grapple tower' and Weyerhaeuser Longview reffered to all thier grapple yarding equipment as a GT).
The early machines had a 'Jillpoke' stiff-leg that ran from mid-boom back to the ground. As with the early Madill 'Yarding Cranes' (which later became the 044) guyline drums werent added or even recommended. The idea was that the weight of the yarder alone would be enough to suffice. It really wasn't.
As loggers always do- they hung out too far, hooked turns way to big, and then complained that the yarder broke or tipped over. All the usual logger stuff.
The machines were developed, tested, redeveloped, and finnaly by the early 1970's Skagit was building 3 grapple-yarding models- The GT-3, GT-4, and GT-5. All were available on tracks OR rubber tires, and were top-of-the-line in the 1970's- which isn't saying too much given what we have seen since.
In the late 70's they re-designed and released the 2 newer models- the 'SY' (swing yarder) series machines. There were 2, the SY-717 and SY-747. The SY-717 was a smaller machine, available on M4A3 (Tank Chassis) or Chapman hyd tracks. The SY-747 was big and only sold on a Skagit 4-axle SP carrier.
Skagit was constantly experimenting and redesigning, hence the A,B,C, and D versions. There was never a GT-5E machine, but lots of D's still do exist.
Mike Walch converted an SY-747 to hydraulic D9G sized undercarriage (using the bottom half of a P&H 2500 130,000 lb shovel I used to own) and had it dredging rock near Wenatchee last I knew. That machine is another story all its own!
Hope this helps some- I'll attach some pics of the jillpoke outfits. I know I have them.
