Thanks to everyone for the responses, links and help.
When going to the dealer to get filters and UDT, he mentioned that they have seen this before. They even stock the parts to correct it.
He said if this was going to happen, the most amount of damage is going to be caused at the mower end of the shaft if it lets go. So I was lucky.
Upon inspection, the HST fins were OK, (thankfully), and only the brake rod was bent. What I thought was fuel was UDT fluid spewing so I was lucky on that one.
The internal fluid screen had no more filings that usual. I drained what fluid was left in the HST, flushed each opening (filter, drain, and screen hole), one at a time with ALOT of
fluid.
I did find some small amout of metal in the drain pan (about the size of coarse ground pepper, about half of one quarter teaspoon total after all the flushing.
I am going to run gently for 10 hours, and change filter and UDT again to see what I get. Could have been much worse, although the extra filings I could do without.
After reading some of the other posts, misery likes company unfortunately.
I agree it is a bad design. I had to get the dents/damage out of the yoke collar since I didn't have more $$$ ($157) for parts. I took it all apart to fix some dents, and found that two of the three ball bearings that click into the PTO shaft were very hard to get to retrack, even though they protrude properly after retraction. While it was off the tractor, I had to use a screwdriver to loosen them up.
Then there was the reassembly of the yoke and collar, which was no problem after the dents were corrected. Then came the reattachment.
I spent a tool throwing amount of time getting the thing back on there. (no tools were thrown) Made sure all slots in both shafts were not damaged.
Finally had to coax the thing onto the pto shaft (gently with a hammer) and it finally locked. I pulled on that thing with all I had to make sure it was on right and locked.
If I had the $$, I just would have bought a new collar. $$$ is very tight now - work put me on disability and I am in conservation mode.
If I had not seen the bearings come back to proper position/protusion I would have not even considered using the old collar/yoke. The problem seems to be their retraction to allow the shaft to seat properly. Every time I tested while off the BX they extended back properly. Still going to buy a new one after my cash crunch.
Lord knows how it is going to act when I try to take it off. Maybe a dealer task, as I don't want to tear up that HST shaft.
That first click I heard before all this was most likely the collar snapping back, but not seated all the way onto the shaft.
Some mowing last night and I am back to normal, no vibration at all like before.
I know it's not good to use the old parts, but I had to.
Everything looks OK other than the small dents on the collar I mentioned before which were easy to fix when taken apart.
Hope this never happens to anyone else.
Thanks again.