I'm piecing together the corporate history of an obscure company from the late 60s. Are there any records? IE public tax returns / registrant records / etc... that I can search online?
I don't know if you can search online, but depending on the company it might behoove you to call the city archives and start there.
If they were in the US, the Secretary of State in states in which they were incorporated or admitted will often have an online search that will show things like filing history, registered agents, assumed names, associated entities, and may have PDF copies of public documents like articles of incorporation/merger and public reports. There's usually a nominal fee (e.g. $1) per search. Records may not go back to the 60's online, but I know in the course of writing insurance for US businesses I've come across PDF copies of documents online going back at least to the 80's. If they didn't do business in the US, just disregard all that.