What is the test for a probation officer to issue a letter to a defendant's probation officer?

And we note further the express holding that a defendant's letter to his probation officer was "not in strict compliance with the provisions of section 1203.2a," and therefore inadequate where, as here, "it did not request imposition of sentence and was neither signed in the presence of the warden or superintendent or his representative nor was it accompanied by the required attestation of that official." (People v. Ruster, supra, 40 Cal.App.3d 865, 871, 115 Cal.Rptr. 572.)