What did Snape claim his source was about Harry's departure date?

Question

At the beginning of the Deathly Hallows, during a meeting with the Death Eaters, Snape let Voldemort know about the date planned for Harry's departure from the Dursley's.

“So?”
“My Lord, the Order of the Phoenix intends to move Harry Potter from his current place of safety on Saturday next, at nightfall.”
The interest around the table sharpened palpably; Some stiffened, others fidgeted, all gazing at Snape and Voldemort.
“Saturday... at nightfall,” repeated Voldemort. His red eyes fastened upon Snape’s black ones with such intensity that some of the watchers looked away, apparently fearful that they themselves would be scorched by the ferocity of the gaze. Snape, however, looked calmly back into Voldemort’s face and, after a moment or two. Voldemort’s lipless mouth curved into something like a smile. “Good. very good. And this information comes—”
“—from the source we discussed,” said Snape.
(Deahtly Hallows, Chapter 1, The Dark Lord Ascending)

He followed in this Dumbledore's order:

“You will have to give Voldemort the correct date of Harry’s departure from his aunt and uncle’s,” said Dumbledore. “Not to do so will raise suspicion, when Voldemort believes you so well informed.
(Deahtly Hallows, Chapter 33, The prince's tale)

But he can certainly not tell Voldemort the truth (Hi, I followed Dumbledore's order, he really wanted to make sure you would trust me - no, definitely no).

And surely Snape cannot pretend to Voldemort he's still in the Order's inside track? Even with a lot of Occlumency, Voldemort could not believe the Order still trusted Snape and invited him to its meetings after he killed Dumbledore and flew with Death Eaters.

So what is it that Snape told Voldemort his source was?

How was it more reliable, how did it let Snape become more important and trustworthy to Voldemort than any other Death Eaters who could spy on the Ministry?

( For instance, Yaxley had heard otherwise spying in the Minitry; the whole thing is too long, but this quote shows how trustworthy the "discussed source" seems to Voldemort:

“My Lord,” Yaxley went on, “Dawlish believes an entire party of Aurors will be used to transfer the boy—”
Voldemort held up a large white hand, and Yaxley subsided at once, watching resentfully as Voldemort turned back to Snape.
(Deahtly Hallows, Chapter 1, The Dark Lord Ascending) )

Some questions are linked to this one, especially How did Snape know about the plans to move Harry? (and its dup) and Why did Snape need to appear well informed?. Mine asks only what Snape told Voldemort in the mentionned discussion: 'I have a good source, it's ***'.

Answer 1

Snape may have told Voldemort about Mundungus

• Snape may have mentioned that he specifically had Confunded or Imperiused Mundungus Fletcher, or claimed that Mundungus was feeded him information voluntarily. There would be little reason for him not to, and a specific name might have increased the Dark Lord's confidence in him. Of course, Snape would not need a specific name: the purpose of this scene is to show that Voldemort has so much trust in Snape that he has private conversations with him away from the other Death Eaters. If Snape had told him he had an anonymous source, Voldemort might not have cared enough to press him.

• With his skill in Occlumancy, Snape might even have successfully claimed that someone other than Mundungus was compromised, if it suited his purposes to do so (which would make him look very "well-informed," as mentioned later by Dumbledore's portrait).

• Mentioning a specific person, particularly Mundungus, could have been risky, since when the deception of the seven Potters was revealed, Voldemort might have wondered why the source didn't know. On the other hand, Voldemort's trust in Snape was enough that Snape could probably have interrogated Mundungus (or his claimed source) himself, and fed Voldemort lies.

Snape had proved his loyalty

• Snape was more reliable and trustworthy than any other Death Eater because of a little something that happened in Half-Blood Prince, not because of the quality of his information

  Snape raised his wand and pointed it directly at Dumbledore. “Avada Kedavra!” A jet of green light shot from the end of Snape’s wand and hit Dumbledore squarely in the chest.

  —Harry Potter and the Half-Blood Prince, p. 33

  
  ***

  “If Draco succeeds,” said Snape, still looking away from her, “he will be honored above all others.”

  —Harry Potter and the Half-Blood Prince, p. 596

  Snape succeeded at killing Dumbledore; therefore he was catapulted into Voldemort's highest confidences, because his loyalty and competence were seen as absolute. It was unrelated to having Mundungus as a source.

• Snape's source was almost certainly more reliable because he had told Voldemort (at least) that it came from within the Order.

• Snape was prepared for Yaxley's contradictory claims, so Yaxley was essentially adding no new information:

  “My source told me that there are plans to lay a false trail; this must be it. No doubt a Confundus Charm has been placed upon Dawlish. It would not be the first time; he is known to be susceptible.”

  “I assure you, my Lord, Dawlish seemed quite certain,” said Yaxley.

  “If he has been Confunded, naturally he is certain,” said Snape. “I assure you, Yaxley, the Auror Office will play no further part in the protection of Harry Potter. The Order believes that we have infiltrated the Ministry.”

  —Harry Potter and the Deathly Hallows