As we have seen in other Questions & Answers, the lineage of Syed Ahmed Al Hasan as goes back to Imam Muhammad Al Mahdi as, and even further through Al Imam Al Hasan Al Askari as.
It is everyone’s right to claim and articulate their lineage to their own grandfather and ancestors, hence, Syed Ahmed Al Hasan as uses this term.
http://vb.almahdyoon.com/printthread.php?t=2501&pp=300&fbclid=IwAR2r__FQc13n7RAkDbOgcVif1FDiO43ZySngaFQ3qDV7ZDdZE0uva-wE6XE
Further, some narrations such as Bihar Al Anwar (vol. 51, hadith 37), that describe the names and titles of the Qaim (Riser).
These fit the names and titles that are used by and ascribed to Syed Ahmed Al Hasan, such as the below…
نا : محمد بن همام ، عن الفزاري ، عن أحمد بن ميثم ، عن عبدالله بن موسى ، عن عبدالاعلى بن حصين الثعلبي ، عن أبيه قال : لقيت أبا جعفر محمد بن علي عليهما السلام في حج أو عمرة فقلت له : كبرت سني ودق عظمي فلست أدري يقضى لي لقاؤك أم لا ؟ فاعهد إلي عهدا وأخبرني متى الفرج ؟ فقال : إن الشريد الطريد الفريد الوحيد ، الفرد من أهله الموتور بوالده المكنى بعمه هو صاحب الرايات واسمه اسم نبي ، فقلت : أعد علي فدعا بكتاب أديم أو صحيفة فكتب فيها .
From Muhammad b. Hammam, from Al Fazari, from Ahmed b. Maytham, from Abdullah b. Musa, from Abdul A’laa b. Husain Al Tha’labi, from his father, he said: I met with Abu Ja’far Muhammad b. Ali (on them both be peace) during the Hajj or Umrah and I asked him: I have aged and my bones have weakened and I do not know if I will later meet you or not? So please make me a pledge and tell me when will the relief come? He said "The homeless, exiled, single and separated from his family, named with his father, nicknamed by his uncle, and he is the one of the black flags, his name is that of a prophet". I asked him, "Please pray for me", so he requested a leather book and wrote something in there.
http://www.aqaed.net/ahlulbait/books/behar51/a4.html