BTW, pics of your spindle show what looks like deep groove radial ball bearings, which probably suck for a mill spindle. That's likely why there are no threads. Nothing to pre-load.
I think the reason the grizzly replacement is has those threads at the bottom of the splines, is because it is an updated version of yours, designed for tapered roller or AC bearings. That threaded portion is likely for the preload spanner nut/nuts that are needed for the preload adjustment of tapered rollers and/or angular contact bearings.
If... well, when..., you replace the spindle, you'll also be doing a worthwhile upgrade, because you'll need a set of tapered roller or angular contact bearings and the preload nuts as well. You may as well order the bearings and preload nuts from Grizzly when you rebuild the spindle, they'll be necessary.
Here is the one piece preload nut from my G0704 CNC build. The 2 cap screws lock the nut onto the threads.
The 2 empty holes are for a pin wrench.
I used Abec7 rated bearings from NSK in this spindle. It has less than .0001" of radial runout measured in the spindle taper.
If your on a budget, I believe Nachi has some p4 precision angular contact bearings for under 100$ a set.
Do your own research though. I can't be certain they aren't knockoffs or something. Since your mill doesn't run much over 2500rpm, I think the polyamide cage would likely be fine. Polyamide melts at about 450°f. My CNC spindle runs 8000rpm, and doesn't get anywhere near that hot.
Nachi Japan 7207 CYUGL P4 Super Precision Angular Contact Ball Bearing: Amazon.com: Industrial & Scientific
www.amazon.com