Your tractor is certainly much different than mine but my power steering cylinder bolts to my front axle - mine has brackets welded on it at approx. the same area as your bolt holes. Maybe your axle is used on a few different models that require mounting the p/s there ? I think I'd agree with Sarg and just keep the threads lubed with something instead of putting bolts in there, I might use some regular axle grease and replenish as it gets washed out over time. I'd also agree that if you do put bolts in there, use the anti-seize compound.