Who wrote the inscription?
I'm not sure.
"From the Marx Bros" suggests secretarial, I would think. Not necessarily but probably.
Boy, that's pretty darn good. "From The Marx Bros." would be secretarial but those sigs. are close, imo. I know I've seen a real photo at some point where the secretary wrote out the dedication like this and they signed.
I have this index card for comparison signed about the time they were tryout touring with sketches from what would become A DAY AT THE RACES (1937):
Thanks for adding that.
I don't claim to know anything about the Marx Bros.' autographs but the style of the inscription seems at least similar to Groucho's autograph.
This was one of a small (4 - 6) group of signed 8 x 10s from the 30s/40s. There was also a Garland and a Sinatra that both looked pretty OK to me, though the realised priced were not huge (£300 - £400 each excluding premiums).