Beautiful bike. Love the darts. I wonder if ‘62 was last year for them as I have two ‘63s sports and neither have them. My ‘63s have same white grips , prop stand, ribbed white cables , and transfers though.
Just a theory but I wonder if your discomfort with the b15 maybe that it was not intended for a upright riding position like a sports. For these I think B66 or b72 ie sprung and wider work better. Clearly with the Benelux and the b15, the owner wanted to put the “sport” back in and maybe with drop bars it may have worked. Maybe put b15 on the shelf and putting something else on if you can’t get used to it. I would be afraid that much proofhide or worse some other softening agents would permanently deform and ruin it. Like to hear what the real saddle experts say.