Marx made tin plated frames in 1939. Unlike the red frames from 1937-38 (which were apparently not used on the tenders), Marx did produce 551 tenders with the tin plated frames. Here is an example from my collection:
Oh! Yes those. I've got a NYC caboose and a CRI&P in the tin frame somewhere. Didn't know those were actually tin plated, or that they came after the red framed ones. I thought they were just not painted.
Thanks for the explanation!