ได้แต่ I797C มาจะกลับไปทำ H1B stamp จำเป็นต้องใช้ I79CA ไหมครับ

Form I 797A is an approval notice for H1B for petition apply within the US in changing status which usually will accompany with form I 797C.  Form I 797 B is the approval notice of H1B for petition from outside the US.
When you come to apply for a visa in Bangkok you must bring both of the notice of action to present to the consulate.