RAF Type 'D' Oxygen masks on US flight helmets

Hello Everyone:

I found a photo of a US Pilot (Colonel Henry 339th fighter group) wearing a B6 type US flying helmet with snaps added to the side to wear an RAF type "D" oxygen mask and wanted to see please if anyone had any info in regards to this practice. He's in a P51D and has an A-14 type mask in the photo but I've only mostly seen "D" type masks worn on RAF B and C type helmets. Just wanted to see please if anyone had any more info on this and photos if possible.

Thank you!

Chris