Rupert did not explicitly require a manner of acceptance in his offer, but he may still have invited a manner of acceptance. See Restatement (Second) of Contracts ยง 30(1).
There are three possibilities here. He may have invited: (1) acceptance by making a promise; (2) acceptance by performance; (3) acceptance by any method reasonable in the circumstances.
The following question and answer sequence leads to the conclusion that Rupert invited acceptance by making a promise communicated in a reasonable time after receiving his offer.