I'm not an expert at all but after retiring two SLX frames - Basso Loto & Eddy Merckx Corsa Extra - I'll throw in my assumption:
This one is very likely pre 80s - so I agree with others here. Aren't the cable guides on the bottom bracket lug the best hint? This brings up one important issue in my view: bottom bracket thread - what is the condition? I hope no bad surprise.
You might already know... anyway I'll just bring it up:
One thing with those old lugged frames: FLEXING or lack of stiffness = softness
It's really a lottery with used frames - unfortunately.
If you're only going to take it out for a 'hipster' cycle it's nothing to worry about. If you want to cycle it properly it might be annoying.
It will understeer heavily and bend so much when sprinting that the rear rim will touch break pads. Don't get me wrong: Depending on rider and weight all italian stallions do that but it definitely gets a lot worse with age.
anyway, keep us updated of your project !