There was no filter, I can hardly imagine how you would mount them on the Elmar, it's tiny.
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If it's the older Elmar, with the aperture tab on the front of the lens, the easiest/cheapest filter solution is to get the "A36 to E39" adapter ring: this slips over the end of the Elmar, and has a clamp-screw (with a shield, to prevent the screw from digging-into the knurled ring) to hold it in place, and provides threading to accept 39mm threaded-filters, or you could add a step-up ring from 39mm to whatever size filters you already have.
The original filter system for the Elmar 35, 50, 90, 135, Summar 50, and Hektor 135 Leitz lenses was the A-36 clamp-on filter; these are getting to be pricey collectibles, plus issues with possible color shift of the filter media, or separation, in the case of Polarizers.
Luddite Frank
PS: I just got home from work and dug around in my kit bags and found my two A-36> E-39 filter adapters, hoping to provide a part number; sadly, the only marking on both is "E. Leitz New York". I have a sticky-note in one of my Leica catalogs indicating that the part # is "ADSUMMI" or 13154, but I'm not sure where I got that info.