I recently purchased one of these from SF, an EN215-1, 15mm ,straight.
They claim to be reversible flow, reversible tail, I assume from that they
can be used in any position and any flow direction, however, they were very
cheap at £4 or so and so I am highly suspicious of their claims.
I need to position it on the return side of the radiator but since there is
no arrow on the valve body, I wondered if someone has any experience of the
least noisy flow direction through the valve.

Looking through the open ends of the valve, it's clear that the operation
depends on a sliding rod to which is attached on one end, a rubber washer
(like an old fashioned tap washer) the washer is held off its seat by means
of a spring wrapped around the rod. So it's normally held open until forced
closed (against the spring) by the temp sensitive actuator.
My question is, which is the best direction for the water to flow
1. Through the brass seat, up and under the rubber washer in the same
direction as the spring pressure?
2. Against the side of the washer thereby forcing it's way between the
washer and the brass seat.
My inclination is to use position 1. however, I'm not sure which is the
flow direction least likely to cause vibration.
I hope my description is clear :-(
The valve will be installed in the bathroom and is likely to be fully open
most of the time, but I thought it prudent to install A thermostatic type to
conform with regs.
Cheers
Tom