Don't quite understand what you mean?
If you have taken the back nuts and olive/rings off a 15mm compression fitting and fitted them onto the end of the showers 1/2" male connections and the 15mm pipe fits inside the 1/2" male fitting on the shower its probably meant to work that way anyway.
Some makers do that.
Incidentally some compression fittings such as Conex use 1/2" bspt on their fittings. So don't think it fits just because the nut screws on. The pipe has got to insert into the 1/2" male fitting like a normal 15mm compression fitting and the top edge of the 1/2" male fitting has got to have a chamfer to allow the olive to compress onto the pipe.
Some male fittings also require a deeper backnut than that found on most ordinary compression fittings.
If the pipe doesn't insert into the 1/2" male then its a 1/2" male iron bspt fitting and intended to have a 1/2" female bspt x 15mm copper adaptor fitted.
Did you notice whether the top edge of the 1/2" male had a chamfer on it for the olive to locate into? If it did then you have probably used it as intended.
