I would agree 100%. The open "cutouts' in the top of the body for the focusing dials would fill with water pretty quickly.
They could have had the R.E.L. Telescope, Observing, Sniper's, with Stand, Instrument CNo.47 fifty years earlier; in test it stood up to six months straight in the rain.
I suppose some kind of sunshade was improvised on the not-so-Swift, so that the objective lens didn't collect bullets?