What would be good selection for daylight as wel as nighttime.?
Depends on what aperture and film speed you'll be shooting at and how you want to balance between flash and available light. There's no telling which setting will be good just like that.
In your place I'd put the flash on a digital camera with manual controls and experiment. Shoot under similar conditions that you'd shoot the A1 with, put the flash on manual and start figuring stuff out. You'll find that at some point you need to do either of these things:
1: Do the math based on the guide number of the flash (=20; hence the product name!) and work out how much flash power you'll need for a subject at a given distance at the aperture and film speed you're working on. You'll have to learn about the relationship between these variables and how the guide number of a flash is defined and used. Plenty of info on this if you Google a bit; give it a try. Here's a good place to get you started including a calculator to play with:
https://www.scantips.com/lights/flashbasics1c.html
2: Use a flash meter - or alternatively, shoot digital and film side by side so you can use the digital as a very fancy flash meter. Frankly, with on-camera flashes, using a dedicated meter is a bit of a pain because you'll have to meter the scene while triggering the flash from the camera from the camera position, so you'd be working with long shutter release cables or some kind of jerry-rigged remote control device etc.
So for all intents and purposes, (1) is the way to go. Practice arithmetic
