I'm not used to optically pure plastic, I would have sworn that
examining the LED side of 2 of the lenses then it was flat faced, but
not touched the surfaces, and they contain recesses.
I now have some rings for placement.
The datasheet does not explain the normal placement. LED soldered to
pcb, a non-optical plastic holder placed over the LED, quite loosely.
The holder is .4mm undersized so forcing the lens into it, then
compresses the other end around the LED with little pip under each
corner, but no use made of the 4 holes in the corners.
I think I've worked out how to adapt those holders for my purposes.