Fine by me if it is unambiguous. 

Agreed, even if it's a little ambiguous. In some cases, the only thing the protestor knows is the sail number. The RC has the contact information. 
Presumably, the RC has made an effort to provide unique sail numbers on the water, too.  Not always successful....

Does the inclusion of a sail number in the description of incident meet the requirements of 60.3(a)?

Yes, it does, in my view. I probably see more forms like this than not. It's fine (Edit: I would confirm with the protestor that the boat mentioned in the incident description is the protestee. If not, then there's a 60.3(a) problem.).

To me, as long as the protestee is able to look at the form and know that they are the ones being protested, then I think the requirement is met.  If the form says "the red hulled boat" and there is only one red hulled boat in the fleet, that may be sufficient.

Rule 60.3(a) Delivering a Protest requires that "When delivered, a protest shall be in writing and identify the protestor, the protestee, and the incident."
As described the hearing request seems to conform with the rule.  

I have no disagreement with the above responses.
However, it could lead to an invalid hearing being commenced if the incorrect sail number was put in the incident description.
I suppose this is the same as the wrong sail number being provided in the respondent's box and when this is established the hearing request should then be declared invalid.
The electronic system used at my club will not allow a protest to be submitted unless there is an entry in the respective fields of Initiator, respondent and incident description so people are prompted to ensure all necessary info is provided.