They look just like the ALICE/M1950 to MOLLE adapter but have a little velcro flap in the middle.   Can anybody tell me how it is used?   Is the flap for the scabbard back strap?

Lol. Nobody uses that

It is not a bayonet adapter, it was born as a Kabar adapter, works with sheaths that have belt loops.
i don't think this is the correct way, i think you have to open the velcro part, slide the belt loop in and close.

That’s correct. You slide the whole thing through the belt loop and then velcro behind the knife or bayonet to secure.

Thanks all.

Both the MOLLE K-Bar adaptor, and the simpler MOLLE ALICE adaptor can adapt legacy gear, which uses ALICE clips or the older belt hook attachment systems to the MOLLE/PALS  Platform.  These were interim devices, brought out at the roll-out of the MOLLE system, when there were many legacy devices still being used.

In practice, ALICE clips will usually mount directly to MOLLE/PALS platforms.  In addition, Users can usually dismount the ALICE clips, and replace them with other attachment methods such as MALICE clips and others.

As noted, these clips are seldom used.  In the event of a user wishing to mount a knife, it is usually better to buy a more modern scabbard which has a MOLLE/PALS interface incorporated from the start.

If one is on an extremely tight budget, then use them as desired.  Just be aware that one will probably replace the item that the adaptors support with a newer one at some point.

FPNI