Wow, it's just stuff I read 50 years ago about Judiazers and a reference to some "large letters" he wrote to them. Galatians is not that large a letter. However, if Hebrews were tacked on the back, then that would qualify as a "large letter" sent to all the churches in Galatia.
Something I found from the DETROIT BAPTIST THEOLOGICAL SEMINARY.
"Essentially, Witherington tries to explain why Hebrews shows up next to Galatians in some of the early canon lists (e.g., in the Sahidic NT). He thinks the order can be explained by the fact that the two documents share many similar features, from, e.g., rare vocabulary (e.g., πηλίκος in Gal 6:11 and Heb 7:4), to the use of similar trads. (cf. Gal 3:19 with Heb 2:2), a similar play on the ambivalence of διαθήκη (“covenant” or “will”; cf. Gal 3:15–17 with Heb 9:15–17—I think he’s wrong about Heb here), and, above all, to their similar emphasis on Jesus’ faithfulness (cf. Gal 2:16; 3:22 with, e.g. Heb 12:2—I’m not sure about the way he reads Gal here). Witherington’s conclusion—“the author of Hebrews may well have been part of the larger Pauline circle”—however, wasn’t as interesting to me as was his failure to note what I had thought was the most conspicuous similarity between the letters (though see the hint on 148): both marginalize the law-covenant with a salvation-historical hermeneutic."
