Arbor rotating with nut when trying to change blade

My Unisaw just started having a problem that has me scratching my head. I was working on a project and was about to remove a stacked dado set and switch back to a combination blade, but when I tried to loosen the arbor nut, it just kept turning and didn’t get any looser. I took a sharpie and marked the arbor at top dead center and then tried to loosen the nut about half a turn, and sure enough the mark I’d made was now at the bottom of the arbor, so the arbor was spinning with the nut.

I looked at the arbor, and there’s a spot to the right of the blade where you can slip a second wrench on it to keep it from spinning while you loosen the nut. I bought my saw second-hand and didn’t get any wrenches with it. I’ve always used a crescent wrench and a piece of scrap wood wedged between the table and the blade to keep the blade and arbor from spinning while removing the nut.

I was able to use a collet wrench to get the nut loose, but that got me thinking that I should probably invest in a proper wrench. The model of my saw is 36-829, and I have done some research online, but there is no diagram or parts list on Delta’s service website.

Can anyone tell me the part number for the wrench I need, or if wrenches from newer model saws are interchangeable?