I have looked at Diplomatarium Norvegicum (
http://www.dokpro.uio.no/dipl_norv/diplom_felt.html ) to look for
examples of the evolution "mr > mbr" - and related stuff - in mid
14th century Jamtlandic dialect. I have found lots of examples of
this:

---------------------------------
Here a quote of a text which I find to good to cut down too much (my
favourite Jamtlandic text in the Dipl. Norv. - a really good story
reminding me about some of the Icelandic sagas):

"Var þat aat vpphafue vidratto þæira at Olafuer ok Hunæfuer varo til
gæst hia Jone iambr hafde þa Hunæfuer fyrnemfdr eina aar ok stak
Olaf j briostid firir ofwan geirwortuna so at Olafuer var lengi krankr
af. sagde þa Hunæfuer at hann vildi eii gort hafua honom en Olafuer
suarade. þat venter ek at þu gerdir þat firir engum ilvilia. baro
þetta ok suoro Biorn iambr ok Jon Þoriss sson at so var ord eftir orde
sem nu er sagt. stod þetta þæira millium ual vm fim aar at þæir varo
ekki sattir en fim vettrum lidnum komo þæir badir saman til Röfsunda
ok var þa Olafuer j kirkiugardenum kom þa Hunæfuer gangande j
kirkiu garden ok talade till Olafs laat eii illa Olafuer min þar sem
þik
þröngir enki vm. Þui nest stak Olafuer oftnemfdan Hunef j briostid
med knifue ok sagde so. haf þetta firir hitt er fyrri var. lifdi Hunæ-
fuer nokora dagha eftir þat en þo do hann þar af. suoro þetta Besse
Berþors sson ok Ketill Skeggia sson at so var sem nu er sagt."
( http://www.dokpro.uio.no/perl/middelalder/diplom_vise_tekst.prl?
b=2523&s=196&str= )

In the beginning and in the middle of the text we find "iambr",
not "iamr" (ON stand. jafnr).
---------------------------------
"Ollum monnum þeim sem þetta bref sea eðr höyra. senda
Lafranz Gunnason loghmaðr j Jamptalande Tofue prester j Fröysöy.
Besse Bærdhorsson ok Ödgisl a Lite q. g. ok sina."
( http://www.dokpro.uio.no/perl/middelalder/diplom_vise_tekst.prl?
b=2551&s=212&str= )

Here we have "Jamptalande", not "Jamtalande" (ON standardized:
Jamtalandi). Evolution: mt > mpt (one would expect 'mbt', but the
unvoiced 't' turns the voiced 'b' into an unvoiced 'p'). I should
note that until late 20th century, Jamtaland was called "Jamplann" by
many of the locals. Quoting the poem "Jamplann" by Rosa Arbman:

"Når småfåglan blister å kvitter,
når natta e ljös som n dag,
når vågan dom gnister å glitter,
då teg'e på vål meg i lag.
Då kjenn je meg rodu te sinnes,
då sjong je, då arbeit je mest,
då veit je vår lykka mi finnes -
då veit je de Jamplann e best."

A raw translated into ON (as far as possible - and I don't claim this
would be Old Icelandic):

"Nær smáfuglarnir blístra ok [kvitra],
nær náttin er ljós sem [einn] dagr,
nær vágarnir "þeim" [gnístra] og glitra,
þá tekr þat á verða mik í lagi.
Þá kenni ek mik ["róðug" i.e. róleg] til ["sinnis"],
þá syngi ek, þá ["arbeita"] ek mest,
þá veit ek hvar lykkan mín finnisk -
þá veit ek þat Jamtaland er bezt."
---------------------------------
"Sældoþa fyrnempðr Sigge ok Ketil Petersson Nichulasse fyrnempdom
ollan þænn lutt sem Peter faðr Ketils fyrnemz hefde att j Dræng firir
fimtan merker jamzskar frialsar..."
( Same document as the one above )

Here we see "fyrnempðr"/"fyrnempdom", not "fyrnem[n]ðr"/"fyrnnem[n]
dom" (ON stand.: fyrnefnðr/fyrnefnðum). Evolution mðr/md > mpð/mpd.
(I don't know why the 'b' is unvoiced here)