This ^^ is basically why it's sealed! When I write a clinic letter I send copies to the GP, other relevant people and to the patient/parent. I'm very careful to be clear, I avoid abbreviations and I listen to what I've said and then carefully read it when the secretary has typed it out to make sure everything is completely clear to everyone the letter is sent to. This takes time - not a huge amount of time but a good few minutes to dictate and a few minutes to check - often I will stay late after clinic to get my letters done so as not to inconvenience patients.
In the case of a GP they have 7 minutes per patient - that's not just to see and diagnose the patient but, if they need referring in that 7 minutes also includes calling the hospital they're referring you to, waiting on hold to speak to the right person, persuading that person you need to be seen, writing a referral letter then telling you where you need to go. As such that letter is something that will be written quickly with as many abbreviations they can get away with, containing just enough info that the accepting doctor knows why the patient is there. If they're writing a referral to an outpatient clinic they will probably dictate this in their own time in the way I've described above but a referral to hospital letter has to be done there and then as you need it to take with you.
Sometimes that letter may contain significant information that the GP hasn't discussed with you, for example if the GP is worried that something really serious is wrong but they are very unsure and want a specialist to rule that out - they may not want to discuss that possibility with you as it may be very unlikely but just something the GP has thought is a possibility, or they may know that they can't actually answer any questions you have as it's not their speciality so would rather someone who knows exactly what they're talking about breaks the news to you. Often in that case though the GP would try their best to talk to the hospital doc on the phone (not always possible) and the letter would be an 'as discussed, thanks for seeing Mrs Bloggs' type thing.
Usually though (and I have seen hundreds and hundreds of GP referral letters in my time!), it's just that it's a quickly written letter which may include abbreviations and things that lay people wouldn't understand and would panic about, as is the case here! Also, it means the letter doesn't fall out of the envelope and you don't turn up in hospital with an empty envelope!
r/v means review. Standard doctor code, nothing more sinister than that!