1601 Spring Bars

For a 1601, which has drilled lug holes, you don't need a flanged spring-bar. 2.0mm diameter would probably be standard to use with a vintage bracelet and endlinks. For straps, the only limitation is whether you can push them through the opening. It might be hard to push 2mm diameter spring-bars through the openings in some straps.

The springbars without flanges are more secure (because they penetrate deep into the lug) and correct for the case. They are no more difficult to use for a watch with drilled lugs. However, you won't be able to use them for other watches that don't have drilled lug holes. In your place, I'd probably get an assortment of 1.8mm and 2.0mm both flanged and un-flanged. And several lengths of the flanged ones (18mm, 19mm, 20mm at least). You always want to have an excess of spring-bars around.

And while you're at it, buy an assortment of the clasp spring-bars with short tips.

Spring-bars often end up flying across the room. You always find them in the end, but it might not be for days.

I use 1.8mm thick bars.