Phase 100 age question

I am getting ready to buy a vintage circa 1970s MXR Phase 100 from a local guitar shop that sells vintage guitar equipment. My question is: 'Is there any way to tell exactly how old it is, using serial numbers or logos, etc?' I'm still planning on buying it, but I'd like to know whether it's legit, also it would be cool to know exactly what year it is.

Thanks for any help you can give ...

Don't know about serial numbers, but the script letter MXR logo was used until about 1978, the later models and reissues use the block letters with the rounded corner rectangle around the letters, serial numbers may be impossible for the earlier 70's models, the company swapped hands to Jim Dunlop around 78, Good Luck.....