A possible case of dissimilation of -r to -l in Written Tibetan

There exist occasional cases of alternations between words with preradical d- and r- in Old/Written Tibetan (OT/WT), for instance dba vs rba ‘wave’, dgu ‘nine’ vs. rgu ‘many’. I am uncertain of the nature (phonological ? dialectal ?) of these alternations. At the same time, rb-type onsets are rare in Written Tibetan and rp- onsets are entirely absent. In a blog dated 19/11/2015 (https://panchr.hypotheses.org/527) Guillaume Jacques proposed that a metathesis has affected pre-Tibetan *rp-, chan..