Each employer shall preserve for at least 3 years:

(a) Payroll records. From the last date of entry, all payroll or other records containing the employee information and data required under any of the applicable sections of this part, and

(b) Certificates, agreements, plans, notices, etc. From their last effective date, all written:

(1) Collective bargaining agreements relied upon for the exclusion of certain costs under section 3(m) of the Act,

(2) Collective bargaining agreements, under section 7(b)(1) or 7(b)(2) of the Act, and any amendments or additions thereto,

(3) Plans, trusts, employment contracts, and collective bargaining agreements under section 7(e) of the Act,

(4) Individual contracts or collective bargaining agreements under section 7(f) of the Act. Where such contracts or agreements are not in writing, a written memorandum summarizing the terms of each such contract or agreement,

(5) Written agreements or memoranda summarizing the terms of oral agreements or understandings under section 7(g) or 7(j) of the Act, and

(6) Certificates and notices listed or named in any applicable section of this part.

(c) Sales and purchase records. A record of (1) total dollar volume of sales or business, and (2) total volume of goods purchased or received during such periods (weekly, monthly, quarterly, etc.), in such form as the employer maintains records in the ordinary course of business.