Zhè fēng xìn shì tā qīnbǐ shǒujì, jiàzhí liánchéng.
This letter is in his own hand and is invaluable.
Tips
usage
手迹 specifically refers to a famous person's authentic handwritten work (calligraphy, manuscripts, letters). Generic handwriting is 字迹 (zìjì); a signature is 签名 (qiānmíng).
