(In AmE, we say "purchase order", don't know if it's the same in HK.)
Do you know the names of the Managers who must sign the purchase order? If so, it's best to address them by name. ("Dear Mr. Jones") If not, you might want to address it:
ATTN: TECHNICAL MANAGER
Dear Sir or Madam: (If you know for sure all the managers are men, you don't have to add the "Madam")
Referring to the attached memo ref.: ABC/06 dated 1 Jan 05 regarding the following purchasing order of building must be endorsed by Technical Manager.
Order No. Building Description
WER-01 XYZ Buy a heater
If you have no comments or changes, Please sign the captioned purchase order.
A copy of the order is enclosed for your information and the original will be sent to your office.