Thanks, typewriterman! It is indeed a black plastic keycap, and posting a photo is great advice (that I will observe next time). However, I must now sheepishly confess that, having posted about this, I took a closer look into the nooks of the portable's bulky black carrying case -- and there was the missing keycap, camoflaged against the inky blackness within! So it turns out I'm not missing a keycap after all . . . and I have a terrific little machine to spruce up. Thanks, and sorry to have wasted everyone's time! (This is, though, a wonderful and friendly board, and I'm glad to have found it!)

Cheers,
Stewart

When you put the keytop back, cut a narrow slip of writing paper the width of the keylever.  Fold it in the middle so that it looks like a staple.  Push the errant keytop over it until it bottoms.  Trim the surplus paper 'shim' off under the keyboard with a sharp blade.  This will stop the loose keytop coming off again, but will make it removeable in the future if necessary.  It's far better than gluing it back on !