New form
Effective July 18, 2011, the previous versions of the IMM 1344 A and IMM 1344 B will be replaced by a new form, Application to Sponsor, Sponsorship Agreement and Undertaking [IMM 1344]. The introduction of the new sponsorship form is another step towards the Citizenship and Immigration Canada’s (CIC) vision of integrated application forms.
Taken from the CIC website.
So fill out the IMM1344 form that they sent you. The letter they sent is out dated.