Bicycle Mechanics - Cassette Compatibility CS-HG50-9

So I searched everywhere for cassette compatibility, maybe yall can help me. I believe to have a 8/9/10 hub on a velocity dyad from Rocky mountain which I have installed a 10 speed 105 cassette with no problems for the past couple of months. I decided to drop down to a 9 speed cassette and bought a CS-HG50-9. I've tried to install and realized the the final 12T cog doesn't go all the way down, the of the spline is beveled so therefore the final cog can't secure on.

I'm thinking it has something to do with compact cassettes and I might have an older cassette?

Is the cassette assembled correctly? This document: http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/CS/EV-CS-HG50-9-1880A_v1_m56577569830608763.pdf
shows that there are 3 spacers (item 14) used for the 12-tooth top gear type. If one is missing it is likely that the lock ring would bottom out before clamping the cassette properly.

Did the 10 speed cassette have a 1MM spacer installed that you didn't remove?

I noted three spacers in the cassette and no previous spacer...I bought the cassette 2nd hand from eBay...is there a chance this cassette could be from a non compact hub compatibility era? It's a 12-25 cassette?

I noted three spacers in the cassette and no previous spacer...I bought the cassette 2nd hand from eBay...is there a chance this cassette could be from a non compact hub compatibility era? It's a 12-25 cassette?
Earlier non-compact freehubs will still accept a 12T small cog. It was the 11T smallest cog that wouldn't fit. As noted, most 10-speed Shimano cassettes have a 1 mm spacer behind the largest cog. It it's still on the freehub body your 9-speed won't seat properly.

I instead picked up a new cassette...a 13-30 Harris Cyclery Cassette / Sheldon Special....It's a 9 Speed and I have the same concern with the other cassette. I've only install 10 speed cassettes on this hub and felt comfortable ....Here's a pic of 8 cogs on and I'm about to place the last one on plus lock ring. It does stay on but it looks like it hangs on the edge of the splines indentions which I don't wan to strip/damage. Am I doing this right?

http://img.photobucket.com/albums/v294/confessor85/20130117_195505_zps50876619.jpg

IF there's a spacer STILL ON the FH body, it would only be 1MM wide and pretty easy to miss if you aren't looking for it.

IF there's a spacer STILL ON the FH body, it would only be 1MM wide and pretty easy to miss if you aren't looking for it.

I'm looking and I don't see it...As it looks, I probably shouldn't mount the wheel?

The last cog on 9 and 10 speed cassettes does not engage the free hub body by much. have you mounted the 9th cog and tried to tighten up the lockring and see how well secured the result is? Andy.

If you are sure there isn't a Shimano metal 1mm spacer behind the cassette (left on the freehub after the 10 speed cassette was removed) then the new 9 speed Shimano cassette will work. There isn't much material for the last cog to grab onto on either the 9 or 10 speed cassette.

http://img.photobucket.com/albums/v294/confessor85/20130117_221635_zpse24aee85.jpg

Here's the shot of the hub...is there a spacer? Maybe I'm not seeing it...

BTW thanks for all the advice

No spacer present.