I am unsure whether you are looking to buy another copy of the lens which produced this image or if you want to buy ANY lens that produces images that look like this. In either event I think you are up against it unless you can narrow the field a bit and suggest which lenses (and possible focal length) it might been based upon what you think was in your possession at the time. Just saying it could have been an Olympus or Pentax does not help much.
The defining thing about the image is that it is low contrast and rather pastel in its colors of course. But the pastel colors and low contrast might just well have been the product of in - camera settings (I think both could be replicated pretty well in Panasonics' menu of options which is reasonably extensive - I have the GX 7). And the "look" of the image may well also be nothing more than the product of slight over exposure (there is a tiny touch of blown highlight in the subject's hair in the second iteration of the image which may also suggest this). It's look may therefore might give few clues as to the nature and identity of the lens. I think this especially is so given you make a point that the light was "was beautiful later fall light" - and yet the image turned out with no real indications of a late fall light (which we mostly think of as being "golden" or "amber" and rich.) Some older lenses I have had are quite sharp - certainly matching this image in sharpness, with low contrast / pastel in its rendering. But, except for some very old lenses having simple or no coatings (i.e. pre war and therefore certainly not Pentax or Olympus) I know of few lenses other candidates that are inclined to produce such rendering without some over exposure and / or fiddling with in camera settings.
The image is quite sharp enough but nothing that jumps out at me or provides other real clues for me. The depth of focus is fairly deep - all of the subject is sufficiently sharp but the background is quite OOF even though rather close to the subject. Especially given that the photo seems to have been shot at quite close range - you appear to be standing above the subject at an acute angle use of a long lens from further away does not seem indicated given the position from which the photo was taken. On a GX8 (m4/3 camera) the field of view would also look like that of a longer full frame lens shot on a full frame camera. But of course that would mean the actual lens would be one a shorter focal length. I do not know what to make of the bokeh other than that it is quite out of focus given how close it is to the subject - suggesting a typical fastish prime lens with its aperture shot near to wide open.
Overall and given these slim pickings, my thoughts run to something in the 50mm range with an f1.4/f1.8/f2 maximum but it's no more than an educated guess and a working hypothesis, and even if true does little to identify the lens without a few more hints from you about what lenses you recall owning at the relevant time. That at least would give others something to start with - they may have owned one of these lenses which matches the rending of this image for example.
As to my lens ownership, I own quite a few M42 Pentax lenses (from very early to late) and a few PK mount ones, but only a couple of Olympus lenses (and none of the latter would seem to fit the bill). One thought does occur though which may help if you are looking to buy any lens that renders like the images shown. My early auto Takumar 55mm f2 and f2.2 lenses and also one preset Asahi Pentax lens are rather low in contrast and of a pallid nature when it comes to color. All are quite sharp enough to qualify, however. But I should add that the preset one is a 105mm f2.8 and I suggest it may be too long given this image was made on an M43 camera and your best bet would be to look further into the 55mm Auto Takumars. Are any of these possible candidates do you think? Other than that I am bereft of ideas.
This link may give you some ideas though the rendering of various images look more contrasty and colorful they are not too far off (and in any event have probably been post processed before posting to the web page.
Auto-Takumar 55mm F2.2 lens reviews and specifications. The Auto-Takumar 55mm f/2.2 is a compact, early M42-mount standard prime produced by Asahi Optical Co. from 1961 to 1963, designed for the Asahi Penta
www.pentaxforums.com
I do not have a suitable photo from the 55mm auto Takumar to hand, but the attached photo was taken with the Preset Takumar 105mm f2.8 of the same era which has similar rendering to the 55mm f2.2. You can see its fairly characteristic rendering. While it has had some obvious post processing I do not believe this has affected color rendering or tones. to any great degree and gives an idea.
