While both lenses share the exact same front optics and glass elements, the ZE version has a slightly wider external metal barrel right behind the permanent tulip lens hood due to the electronic Canon EF aperture communication bus. This extra millimeter of barrel thickness on the ZE variant is exactly why your "ZT 15-150" ring is physically stopping 1mm short of fully locking into place. 

The Real Problem: Ring Incompatibility

 box explicitly reads ABS-CORE ZS15, but the ring inside is engraved ZT 15-150. 

• ZT stands for Zeiss Tourist / ZF.2 (Nikon specific indexing).

Because the ring has an uncompromising, zero-tolerance all-metal friction fit, no amount of heating or careful alignment will make a ZF.2-dedicated ring stretch over a ZE housing without permanently gouging the paint on your classic lens.

"WonderPana Absolute ZT 15-150" ring you have will fit the Carl Zeiss Distagon T* 15mm f/2.8 ZF.2 lens (Nikon F-mount). 

The Specific Compatibility Rules

• It fits: The ZF.2 (Nikon) manual focus version of the Zeiss 15mm f/2.8. This version has a slightly slimmer rear barrel sleeve just below the built-in petal hood, allowing this ZT ring to slide all the way over it.
• It does not fit: Your ZE (Canon EF) version, because the body housing behind the hood is roughly 1mm wider to make room for Canon's internal electronic aperture motors.
• It does not fit: Newer Zeiss Milvus 15mm f/2.8 lenses, which have a completely redesigned, bulbous rubberised exterior chassis.