According to Michael Krogsgaard's research (see link in Appendix F) this session was specifically for overdubbing "4th Time Around", and the musicians involved were Kenny Buttrey (drums) and Charlie McCoy (harpsichord). There is clearly no harpsichord on the released track, but it's quite possible that the harpsichord idea was dropped before the final complete take.  Michael Krogsgaard states that this session took place after the release of  Blonde On Blonde, and that the overdubbed track was never released.  However, at the time Michael wrote this it was widely believed that the album had come out on May 16, a month before the overdub session; and of course, no version of the song containing a harpsichord has ever appeared.

It's certainly clear that the overdub was done fairly late in the production process, as at least two mono mixes and one stereo mix had already been made prior to this, as evidenced by the Canadian and French mono editions and the original US stereo version.

Furthermore, if the overdub was not intended for use on the album, then we lack an explanation for why the session was held at all.