As already noted by others, this mk 1 ED scope will not work with most of the eyepieces that fit later Fieldscopes, including all WF/MC/DS fixed eyepieces and all zoom eyepieces. The one exception is that the 24x WF/MC eyepieces will fit, and either the WF or MC would be very nice if you could find one. They provide the same true FOV of the 20x regular fixed eyepiece but at a slightly higher magnification. That eyepiece is also very nice, with good eye relief for glasses. It is the one I use with my mk 1 nonED Fieldscope, which works beautifully for me as a dedicated car window mount scope. The other regular (nonWF/MC/DS series) fixed eyepieces are optically excellent but have narrow FOV and poor eye relief.
If you want to use anything other than a 20x (or 24x) fixed eyepiece, get the Fieldscope II ED instead. Or get the Fieldscope III, which is fully multicoated, better waterproofed, and has a built-in hood. I think the version II is the best value because they regularly come up on eBay for $300 or less, in pristine condition, and including one or more eyepieces. The fixed 30x WF is my favorite on that scope, but any of the traditional (i.e. non Monarch or EDG) fixed or zoom Fieldscope eyepieces will fit, and it is available as straight or angled.
--AP