how do I what size prewound bobbin your machine takes

[broncobuster]

With all the bike rallies i do i would like to get the prewound bobbins to save some time. anyone know how you find out what size prewound bobbin your machine takes or where to find the conversions. ive got the 29-4 size n i found my landis but i have upholstry machines i use n i cant find the model number on one of my machines. most places wont give ya sampling of sizes. so thought that there must be a chart or something somewheres.

[Art]

Give us some pics, maybe someone will recognize it.

Art

[broncobuster]

I have about 5 or 6 different machines i just like to know how you know what size prewound i need like an L or an M. are they certain size? or width? do you measure the shuttle to know what size it takes? what thickness the bobbin is? prewounds go by letters how do i know what size each letter is? the one i use the most is an ANKER its my baby been useing it for 30 yrs with no problems. and has lots of miles on it from doing shows. just was wondering if anyone knew how the letters go for sizing. thanks