Is it wrong...

[vestlenning]

HW35's are something special on their own.
The transfer port size of the leather seal models was about 4,2 mm.
If nowadays you like to switch such an old system to a synthetic seal, easy-peasy with an adapter, the TP-size should consequently be reduced to about 3,0 mm.

Easily done by gluing in a brass tube like the ones available here:
http://wasana-airgun-tuning.de/shop/sonstiges/
Please scroll down little bit to:
Messingröhrchen 4 x 0,5 x 30mm für HW35 alt

Mine is a 1980 leather, now plastic piston seal. Nothing done to the TP but the rifle works just fine, consistent at about 220 m/s with JSB Exacts.

http://barx.org/airguns/?id=hw_35