The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 4, 'Abbreviations'
4.4 Use of Full Stop
A contracted form of a word that ends with the same letter as the full form, including plural -s, is not followed by a full stop:
Dr, Jr, Mme, Mr, Mrs, St, vols
but note the exception ‘no.’ from Italian ‘numero’, plural ‘nos.’. Other abbreviations take a full stop and are followed by a space:
M. Dupont (Monsieur), Prof. J. Jones, l. 6, ll. 22–28, p. 6, pp. 106–09, vol. xix
In lower-case abbreviations for expressions consisting of more than one word, there is a full stop after each initial:
a.m. (ante meridiem), e.g. (exempli gratia), i.e. (id est), n.p. (no place [of publication]), n.d. (no date [of publication])
Full stops are omitted in capitalized abbreviations or acronyms for:
(a) standard works of reference (italicized), journals (italicized), or series (not italicized):
DNB, OED, ABELL, MLR, PMLA, TLS, BAR, PMHRS, PRF, TBL
(b) countries, institutions, societies, and organizations (none of them italicized):
UK, USA, BL, BM, UNAM, CNRS, ANTS, MHRA, MLA, UNESCO
In bibliographical references, use MS, MSS (‘manuscript(s)’). In normal prose text the word should be written out in full.