Track frame holes D4?

While showing a tractor to fellow member and potential hoarder, we couldn't help notice on an RD-4 that the track frames did not have holes for attaching blades, etc.
Unless our eyesight was very bad, we did not see any corks, nada, smooth as a babies rump. 
Do I have an odd tractor? I was under the impression that the entire crawler line up had holes tapped and threaded for accessories. Or was there a period that the factory did/did not offer that, say optional? Kind of a silly question, never noticed that before. JM

 [attachment=76889]Screenshot_20240623_101311_Chrome.jpg[/attachment] [attachment=76890]Screenshot_20240623_101311_Chrome.jpg[/attachment] [attachment=76889]Screenshot_20240623_101311_Chrome.jpg[/attachment] [attachment=76890]Screenshot_20240623_101311_Chrome.jpg[/attachment]Track frame holes were optional on the RD4.  I recently restored one that didn't have any.  My recollection is that there were a couple different opinions for the hole pattern as well but i could be wrong.  It is listed in the parts book.

Thank you! I learned something today. I appreciate you posting photos/information from the parts book. I won't have to look now! JM