Anyone know about sizes for pre-slim shirts??

So, I've got a Gustin Oxford, size small, that says neither Classic nor Slim, so I'm assuming it was made before the slim option (??) Am I correct in thinking this? If so, is it the same as the Classic measurements they have today? If I'm incorrect in my assumption - how can I tell if it's a Slim or Classic?

Background: the shirt fits me perfectly, and i want to buy another Small but I don't know if I should go slim or classic

Thanks in advance for your help!