Hi!
Your observations are right on!
The Rules of Golf prohibit adding anything to the shaft that makes a bulge, and the handle can have tape added but the shape has to stay the same from top to bottom of the handle.
Weight can be added inside the shaft. Sand or salt or other powder can be poured into the shaft, so the weight concentrates in the bottom near the hosel.
Now weight can be addedd at the top of the shaft. A company called Balance Certified Golf (BCG) offers a system for weighting the top of the putter shaft. Their website is
http://www.balance-certified.com. This extra weight alters the vibrational node of the putter by moving it up the shaft from just below the grip material to about the middle of the grip. The node is the point along the shaft where the vibrations calm down. The result is a feeling of solidity at impact.
Cheers!
Geoff Mangum
The PuttingZone
Golf's most advanced and comprehensive putting instruction.