A formal letter-closing phrase used to apologize for not mentioning something earlier in the letter. It conveys humility and is typical in Japanese written correspondence.
Finally, I wish you all good health.
Last but not least, I would appreciate your reply despite your busy schedule.
Ateji variant using 乍ら for ながら; rarely used in modern writing.
Compound of 末筆 (まっぴつ, 'end of a brush/writing') + ながら ('while, though'). The phrase literally means 'while it is the end of my writing', used to humbly introduce a final remark.