There are two "Felso" adverts on ebay at the moment.
The box give it away that "Felso" is a product of Fels and or related to Fels soap.
Advertising/marketing by "sight" was not unknow in the 1950's and the easily sighted shape of the Fels logo and repeating "Fels" type writing (in red on the angle) were the same on Fels soap bars.
Finally one can see in small print, it clearly states "product of the Fels company".
As suspected Felso was Fels answer to modern detergents like Tide which were rapidly displacing soap for laundry and other household cleaning chores like washing up.
Then as today one product is marketed for laundry, and dishes. In another advert (also on eBay) Felso is marketed as "why by three when only one will do". An obvious ploy for housewives seeking to make their housekeeping allowance go further.
Notice Felso advertises it contains "nylon whiteners" and gives a "whiter, brighter" wash. This would indicate Felso like the vintage bars of Fels soap in my larder contains some sort of OBA. Am willing to bet "Felso" is a white version of the vintage Fels soap bars. Give away would be the claim to "fluffier" and "softer" laundry. Pure detergents like Tide do not make that promise, and washing laundry in soap does have a built in "fabric softener" affect. Indeed my vintage box of Instant Fels states it contains "built in fabric softener".
L.