http://legal-dictionary.thefreedictionary.com/National+Firearms+Act+of+1934 is a summary of the original version of the act, it just applies to machine guns and sbr/sbs. it defines a sbr as less than 18”, which was later changed to permit surplussing of M1 carbines and the addition of DD with GCA in 1968. I thought I’d read silencers were added in 1938.
I read the freedictionary source. It seems compounded from a number of post 1980 sources that relied on then current mythologies, rather than original sources from the period.
Every scholarly source I can find includes silencers and handguns, generally, in the proposed bill. Handguns were taken out because Congress objected.
If you study these laws, you find that the “reasons” for passage that are publicised are often just propaganda. For example, there was not significant amount of crime to warrant the ban on the interstate sale of switchblade knives. It was all ginned up by a Congressman from New York City and the media.
Thank you for your efforts in finding a source.