I have both the FA77 and the K85 (and in fact, most of the Pentax 85 )
They are different in real world use.
The 77ltd is a more versatile lens in that its shorter focal length makes it much more usable indoors as well as out.
The engagement distance is shorter for the same framing.
Add to that, its got 50mm like characteristics in that its very sharp once stopped down 1 stop and sharp all thru in the f5.6-f8 range (50mm like sharpness)
The K85/1.8 is 85mm and hence the engagement is that of any other 85mm lens.
In other words, you stand further from your subject than the 77mm for the same framing.
This is with the 77ltd.
I simply asked if I could take a photo to the porter at the fishman's jetty, he acknowledged with a nod, I lifted the camera, shifted a little for framing and snapped the photo.
With the longer working distance of the 85mm, I would have had to ask him to hold on a bit, walk one or two or even 3 steps back (and the moment might have passed, or he may go about his usual business)
Its such practical aspects in use that seldom gets mentioned when all people try to compare is DOF, bokeh, etc.
Somewhat similar situation here. There isn't that much to move back in the temple area, I framed the shot with that short moment of eye contact and took the shot. With the 85mm, I might have had to press myself to the wall to get the framing or even move to another spot, loosing the moment.
77ltd is also much smaller, almost a negligible addition into the bag as a short telephoto option.
That said, the K85/1.8 is a great lens too.
Basically you need all of them (which is why I have so many )
With the K85/1.8
Yes, I had to say .. "wait.. wait.. as I moved a step or two back to frame"