2 tools?

[Jasper Odell]

I have just rummaged thru a box of old model steam engines & steam toys that I bought many, many years ago. Nothin' of any real value (checked similar stuff on ebay).
My problem is when I see a missing part, or don't know how to identify a bolt, or nut, or a fitting .... well, how did you folks get started in identifying the size & type of nuts/bolts/fittings on this tiny stuff ?
The matter gets complicated by weird "standards" , ( I could never spot a Metric from a British Whitworth without first ruining the threads)!

I have done backyard 1950's Chevy restoration, but my simple thread pitch gauges and telescoping micrometers are way to BIG to use on these model steams.... and frankly, I am simply used to working with SAE seldom smaller than 1/4-20.

Can anyone recommend 2 tools/gauges to get me started "measuring" and ID-ing little nuts and bolts are on these "mini" machines?

(I ask for 2 to get me started cause thats likely all I can afford right now!)

{PS ..Is there some sort of chart I could post on my wall that shows pics of the various smaller nut/bolt/ &pipe fitting types? Maybe a reference modeling book too ....??}
Thanks for the help!