Same entry (though see n. 1 below) in the Synagoge, Photius' Lexicon (mu290 Theodoridis), and the Lexica Segueriana. The first glossing word, mesaipo/lios, is itself glossed in mu 648.
The neuter noun mesa/gkulon, literally "mid-loop", refers to a javelin with a thong (loop) used to hurl it, and is mentioned in tragedy, comedy and historiography (see LSJ s.v., and alpha 251). The present headword mesa/gkulos, evidently related to the adjective a)gku/los (cf. alpha 253, alpha 254, alpha 255), does not occur outside lexica.
In Hesychius mu892-893, mesa/gkula (sic) is directly followed by mesaipo/lios. As Bachmann and Nauck noted, lexicographers after him appear to have conflated the two entries.
[1] Theodoridis on Photius s.v. prints this as two words, mesos polios.