Those were some good questions.  The creeper drive I believe was used for mounting a concrete/rock crusher on the back of the tractor (If you google "Allis Chalmers I-600" you will see a U-tube video that will show a rock crusher on the back of I-600).  The 615, I-600, D15 Series II are pretty much the same tractor.  The transmission parts in 615, I-600, H4, HD4, D15 Series II,  are all the same.  The 615, H4 and HD4 share the same engines gas and diesel.  Your 615 has a stuttle chutch opition which means the PTO speed will run between 700 RPM to 850 RPM.  The shuttle clutch opition causes the PTO speed to increase above the standard 540 RPM.  So in a nutshell it does not matter if you change the shaft from 540 RPM or to 1000 RPM.  The speed will stay the same at roughly between 700 RPM to 850 RPM with the tractor running at 3/4 to full power.  It was the design of the tractor.  The only way to make the tractor run at 540 RPM is remove the stuttle clutch opition and put in the power-director opition.  Making the change is somewhat intense.  I have I-600 and a while back I thought about putting in the power director because I had to split the tractor to fix the shuttle clutches.  I was told at the time, Allis Chalmers was trying to develop a tractor that could run a 540 RPM and 1000 RPM but they developed a tractor that really runs at 800 RPM.  I got a call once from an engineer, who's now in the nursing home, who's job was developing test tractor's for Allis Chalmers.  He told me there was a Allis Chalmers I-400, test tractor, that was 4 wheel drive, and had the shuttle clutch opition (same as I-600, 615), plus the hi/low range (.5 mph) was the slowest gear, a clawer gear, and was a diesel.  A smaller tractor than I-600, 615 but these opitions as the time might have changed the market for Allis Chalmers.  The year was 1969 an Allis Chalmers decided to go overseas to have these smaller tractors built.  I asked him where do you think that I-400 might be today, he said men from the corporate office, came and took the tractor away, and he thinks the tractor went over seas in which they studied the tractor to help builted AC 5020 because some of that technogoly of the 5020 four wheel drive came from that I-400 which he help developed.  More information than you wanted know but somewhat interesting I think.

My H-3 with the shuttle has standard 540 PTO but the rear PTO housing is different than the D-15/ I-60,600,615 and has extra gears to standardize the PTO speed.

That is some amazing information DseriesIII.  That would have been the perfect tractor for me, shuttle clutch, hi-low with a very low first gear for tilling, diesel and 4x4.  I really wish they could have build this tractor and offered it at a competitive retail.  I wonder if there are any pictures of this tractor?