Roy,
I have never seen the 38ET light listed for the Dragonfly. The 37ET was the original fitment as stoplights were not mandatory when the Dragonfly was produced. I have fitted a twin filament bulb into my 37ET light by using the contact assembly from a scrap Lucas light, and then just filing off the staggered pin from the side of the bulb to make it fit the original bulbholder.
I believe the 38ET only became available at a later date - to meet the demand for stoplights to be fitted.
Regards,
Eddie.