The Club 420 class rules only state:
"4.7 The length of the forestay, including any extensions, which must be metal, shall prevent one-half of the major dimension fore/aft of the mast from going aft of the mast partners when the jib halyard is released."
Looks like you need to put the mast in with the forestay and make sure it doesn't let the mast rake itself out beyond the partners. It could also be that the previous owner planned to replace the forestay and found something he figured he could cut down to fit. Better to test it for the rake before you cut anything, of course. Based on the rule's wording it also sounds like the jib and jib halyard are used to keep the mast forward; the forestay only comes into play when the jib is down.