Finding similar model numbers do not mean identical parts.

Recently I undertook a project to rebuild a mower deck from a 42” Toro Z420 machine. Aside from rust issues all the bearings in the spindles and idlers were bad or going bad. Murphy assisted by predicably helping me to snap off 3 of the 8 bolts attaching the spindles to the deck body. I tried drilling out one of the snapped off bolts and of course it was a hardened bolt which defied my efforts to drill thru in order to use an easy out. After the failed bolt extraction, I decided to look at replacing both housings.
Being the clever fellow that I am, I went on line to research parts to repair the deck. The idler pulleys were simple to obtain because they are a single assembly. Hoping for a similar experience with the spindles I found Toro does not even list the spindles as a complete assembly with a part number. They only list individual components of the spindle for sale separately. This assures that DIY’s can’t find compatible assemblies even if they are made by after market venders.
I did the reasonable thing and searched the net for replacements for the individual components because I was trying to complete the project
My research for the Z420 yielded part numbers for the spindle housing 88-4510 and bearing shield 80-4360 along with the shaft 80-4340, pulley 88-5330 and bearings 38-7820. The dealer price of the pulley is $104.12 and the shaft $42.89. The price for the housing alone is $83.99 and bearings $12.64 each.  Total for those 4 items was north of $150 not counting the pulley.
My big mistake was assuming Toro would use the same basic parts for its next model Z4200. The part numbers for the bearing shield, housing and bearings were the same. Since I already had 2 serviceable pulleys for the Z420 which I was able to salvage from the original deck, I never bothered to price or compare either the pulley or the shaft for the Z4200
After looking at the numbers I was getting I naturally went looking for alternatives. I came across a Stens aftermarket complete spindle assembly. The description said it was compatible with the Z4200 so I ASSUMED incorrectly that because it was compatible with the earlier model Z420.   
Cutting to the chase after receiving the Stens spindle assemblies we discovered that the new spindle shafts were to short to be able to safely tighten the keeper nuts in place using the pulleys salvaged from the original deck. Comparing the new shafts to the original shafts I found that the Z4200 shaft was ½ inch shorter than the Z420 shaft. The Z4200 pulleys are about $60 cheaper that the Z420’s as well as being ½“ shorter.  Further research found that Toro had made the sales price of the Z4200 machine $600 cheaper than its older brother the Z420. Somebody in the Toro sales department probably told engineering to make the Z4200 much cheaper than the Z420 and the newer spindle assemblies were one of the ways they did that.   
Now I am stuck with 2 aftermarket spindles that don’t fit my application and the prospect of again trying to extract the hardened bolts snapped off in the aluminum housings.
Wish me luck.