You know... I thought I'd do something a little different so I went into an old box of scorecards and pulled them one at a time to see how far I'd go before encountering a slash hole. On my seventh draw, I pulled the card from Rancocas, a public RTJ track in South Jersey where #4 plays 370, 405, or 420 from the different tees to a par of 4/5. Thinking back on that hole, it's straight away and generally plays with a little cross wind, if any wind at all. I'm proud to say that my four on the card for that hole isn't circled like a birdie that it wasn't. It's a nice par four but I see NO reason for the slash. I guess it was a tad sexist of me to assume that slash holes gave the men's and women's pars. Maybe that's the intent in some places but it's not like the rating is 70.0 / 71.0.
I like the idea that was proposed earlier that it is what it is... a certain chunk of real estate to be negotiated so don't worry about the par. When I was just starting out, my dad was probably equal parts dogmatic and incorrect. He told me that up to 249 was a par three, 250 to 450 was a par four and above that it was a par 5. Par is not a simple function of distance.
I don't know why the slash but let me start digging again and see how many cards in the first 50 have slash holes. Okay... after 50 cards, Rancocas was the ONLY club with a slash hole so I'm going out on a limb to say, it isn't a common convention. Maybe it's a modern day shorthand to describe the old custon of assigning a par of 3 1/2 to a particularly difficult 238 yard hole?
The only place where slash holes must be defined one way on the other would probably be in a Stableford competition.