I got a fun surprise for the New Year, my PF cert for a RS138 var went on-line. I was going though some dups and came across a RS138 that didn't seem to have a watermark. I thought it was old paper but it had a single blue silk thread visible on the back. Though not listed in the catalog, it looked like experimental silk paper to me, and some friends, so I dropped it off at PF last summer. It came back positive back in November but it took until now for the cert to be posted on PF's site: #562545.
So look through your RS138s and let me know if any more "e"s are out there. :)
The cert only has a photo of the front, so here is the back, which is the interesting side anyway. The lone thread is center top.