Those of us who were members of the former CompuServe Literary Forum witnessed the birth of Diana Gabaldon’s Outlander series. She uploaded snippets of the work in progress, encouraged discussions about them, and later when she became a successful author continued to support the forum and answer our questions about the art and craft and business of writing novels. Her gracious support of other writers included her writing a blurb for my novel The Sun Singer.
In our craft discussions, she and I disagreed on one thing. And that was, should the author stop writing while completing a gap in the research, or should s/he continue writing and fill in the correct information later?
She said: “Keep writing.” I said: “Stop writing.”
She argued that when the author was on a roll, stopping to fill in historical or other information would simply derail the flow of the novel and the author’s daily writing process.
I argued that writing while something is still unknown could very well send the novel down the “wrong road” and necessitate a lot of needless rewriting later.
She preferred to put a “placeholder” in the manuscript, reminding her that she still had some facts to verify before submitting the book to her agent.
I preferred (and still prefer) to know the facts–whether they apply to history, geography, customs, or whatever–before I write the next scene or chapter.
It goes without saying that her Outlander series of books–and their spinoffs–have been infinitely more successful than my novels. So, I suggest you follow her advice and keep putting words down on the page even if you’re not finished verifying your information.
The fact that I’m eight years older than Diana doesn’t mean that I have more wisdom. It simply means that I’m eight years more set in my ways. I’ll freely admit that as I continue pausing my writing while checking my facts.
If you’re not set in your ways, putting a placeholder in our MS is probably the smart thing to do until you have time to look up what you still need to lookup.
My novel-in-progress, “Weeping Wall,” sat for several months while I verified the geological information I needed in the first paragraph.
Malcolm's Round Table 
I’m with Gabaldon: keep going. I never cease to be amazed at how I am nearly always right when I plough ahead and check the facts later. It’s not something I’m smug about. I guess the general reading and thinking I do in preparation for a book means I know what shape the story is going to be. So the unchecked details are less likely to be way off beam. They get checked in the end. And I do preen a little then …
I have no idea where the story is going until I’m writing it, much less what kind of shape or ending or subplots it’s going to have.
Hmm, I sort of do both. I try to do research in advance, especially of the time period, but if all the scene needs is [correct word for coin denomination] then I’ll insert square brackets and keep writing!
So your process might include the best of both worlds.