"M" category Visa requirement. Multiple entry with 6 months validity. Each stay 60 days

Dear, in case that you meet the following requirements, you don't need the Red Stamp Invitation Letter:
In case of any of the following situations, the applicant can be exempted from "Invitation Letter(TE)"or "Verification Letter of Invitation",only submit the ordinary invitation letter issued by the inviting entity or individual in China:

(1)For the applicant who is working in the central government of Sri Lanka, providing a letter indicating his/her employment status issued by Ministry of Foreign Affairs of Sri Lanka.

(2)Providing original and photocopies of Certificate of Incorporation, personal bank statement of at least 2 million rupees deposit or a booklet of share certificates of at least 2 million rupees in the latest three months or more.

(3)For the applicant who have good records of visiting China(either of the following conditions):

(a)Providing photocopies of previous M or F Visas to visit China for more than two times and good records of visiting China within recent two years, as well as photocopies of entry and exit stamps.

(b)Providing photocopies of previous visas to China for more than three times and good records of visiting China within recent two years, and providing relevant records of entry and exit, proof of commercial activities in China,such as contracts and current accounts etc.

As you mentioned you have been to China before more than 6 times, it's very likely that you do not need this letter.