Most collapsible lenses that come with Russian cameras do not have lens caps. The caps were lost decades ago, and were never produced in sufficient quantities to replace the lost ones.
After a long and frustrating search we finally found a generic lens cap for the collapsible lenses with a 36mm outside diameter.
This includes FED 50mm (AKA Industar-10), Industar-22, Industar-50.
The lenscap is a push-on type.
The lens on the picture is for illustration only and is not included.
Please note; the lens cap is not particularly deep, approximately 4mm. This means that it may not fit ideally on your lens. The height of the front rim of the lens varies, depending on the make, model, and age. On some FED lenses this cap covers the front rim completely, and on some later Industar-50 lenses it only cover 2/3 of the rim. In all cases it does the job of protecting the front element of the lens.