See the Sailing Texas website. In the classified section one will find several pics of Venture 21s. You will see a couple of different standing rigging designs in the pictures alone.
Mauri Pro Sailing website lists 4 different rig configurations possible over the production run of this model of sailboat.
You will need to educate yourself about the type of standing rigging you have. If the forward wire, called the forestay, is attached at the very top of the mast, you have a masthead rig. If the forestay attaches at about 7/8s or 3/4s of the way up the mast, you have a fractional rig.
If you only have a single side wire on each side, with a single forestay, you have about the most simple rig arrangement possible. If you have two short poles that are attached to you mast that hold the sidestays away from the mast, they are called spreaders. If the spreaders form a slight "vee" and point aft slightly, you have swept spreaders. A forestay and two sidestays with swept wires form about the simplest and least expensive standing rigging arrangement. It is also the weakest rig, but the entire line of Macgregor sailboats are manufactured with the least equipment that will do the job.
A backstay can strengthen your standing rigging, especially if you have a masthead rig. It can be used to tune the forestay tension. In a fractional rig arrangement, the backstay can be used to depower the mainsail. In all standing rigging arrangements, the backstay can counteract the stresses imposed by large headsails (genoas, spinnakers). If you want to install a backstay on a sailboat that has never had one, select carefully the location of your backstay chainplate installation. Many sailboats have backstay that are offset from the centerline of the hull for clearance issues.
The attachment point where the standing rigging wire terminates at deck level is called the chainplate. On Macgregor sailboats, this is often a piece of stainless steel with a hole in it. Chainplates are often attached under the deck or to the side of the hull. The adjusters for rig tension are found attached to the chainplate. Macgregor often installs an adjuster that uses multiple holes to be indexed with a rigging pin. One rigging pin goes through the chainplate hole and the bottom of the adjuster. Another rigging pin goes through the selected hole in the adjuster and connects to the wire in question. The rigging pins are held in place with cotter pins or circular rings.
Please describe your standing rigging exactly. John S