Fungus, it is.
Hello,
I have dealt with this in my 3,5T before, and it is actually rather straight forward, in the viewing lens. Unscrewing the lens group shouldn't even need you to remove the front cover from the TLR: if you wear surgical gloves, and are careful, you should be able to take off the front group, then hand dismantle the lens. make sure you lay out each lens in the right order, and right direction, on soft tissue paper or chamois.
Oddly, I have used a combination of tow domestic substances to effectively remove fungus. Hydrogen Peroxide kills it instantly, but does not remove it. so you can use peroxide to clean out the metal barrel, and wash over the lenses with a q-tip. then, get plain white vinegar, and float that on the affected lens. leave it for about half an hour, then try wiping gently with a clean q-tip. it should just slide off.
clean, blow dust off, reassemble, and take pictures.
if you haven't got one, i have a pdf copy of the Rolleiflex TLR service manual.
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