The De Lisle was made in very small numbers, and was intended to be a suppressed weapon (although not all had the "silencer" fitted), something useful for a commando team to take out sentries with. I have not seen any account of them actually being used in combat, but it may have been. IT was never intended to be any kind of "all in one" but rather a highly specialized package.
Not sure just what you are looking for when it comes to "lesser known" but still serving...most of the lesser known weapons are lesser known because they were withdrawn from service (or outright abandoned).
The Reising SMG was not well liked by the troops who used it in combat, and was withdrawn after a relatively short life, but it was carried (and maybe used?) by prison guards in several prisons for some decades after its withdrawl from combat in WWII. Would that qualify as "serving WWII - up"?
Of course, any Reising still in "service" today would be a huge rarity (and relic) as generally even prison armories are now stocked with much more modern arms. There are even a few small police depts around the country where they have a Tommy gun, or maybe two, still on the books (and maybe more that are not "on the books"
) still active, even though they may not have been used in decades. The Tommygun is famous, so that would not qualify as lesser known.
The Johnson rifle and LMG are lesser known, but when they passed out of military service, they went to the civilian market when and as possible, so none of them continued to "serve" after WWII. Although there may be a scattered handful around the country still in police lockers.
In general, if the military dropped it (for whatever reason) it usually wasn't popular with anyone else.