TL/DR - 2019 JMJM's have thicker necks, whiter bodies, yellower-more satin necks. Neck pocket and pickguard measurements ever so slightly different. 2019 pickguards are more yellow/less copper.
Background:
The J Mascis Squier Jazzmaster has been a favorite of many for its incredibly value. More than any other feature that gets it mentioned - is the neck. People that love the neck generally love it because its C shape, and the jumbo frets. Personally, I've bought several of these and currently have examples (of at least the neck) from 2015, 2017, 2018, and now 2019.
I recently used my Musician's Friend Backstage Pass points to buy another J Mascis Jazzmaster to part out for a project. When it arrived, I immediately noticed that the neck felt entirely different. Yes, all necks feel different even on the same model - but the general characteristic of the JMJM necks I have from 2015-2018 are generally the same (very thin and the jumbo frets feel obvious).
What felt different on the new 2019 JMJM Necks?
The neck feels fatter, chunkier, and wider than all of my earlier examples.
Maybe it's just the one you got or the setup?
That's what I was hoping for but it is not. I measured it and also went to my local GC where they have a 2019 in stock. The new ones are different. Also I took some rough, but fair measurements. See photos below. Additionally, I tried to swap the necks because the 2019 body is about a pound lighter - but the neck is so wide - it would not fit into the neck pocket.
What else is different?
2019 pickguard - more gold, less copper (yay!). Also measurement is ever so slightly different. I swapped pickguards and the screw holes did not align perfectly.
2019 body - finish is whiter, less sandy/gray
2019 neck finish - the new ones are siginificantly yellower and more satin
2019 nut - the older JMJM's had a grayish rounded nut. The new ones are starch white and rectangular
Fender/Squier's Response - I called the customer service line to ask if there were changes because the 2019 I got felt different and their response (verbatim) "We are under no obligation to advertise any changes to our catalog.". It was an odd response but according to my sales reps at MF, GC, and Sweetwater - Fender is appearantly known in this industry to be shitty to customers.
Photos
Note: I swapped the pickguards in these photos. The 2018 PG is on the 2019 and vice versa.
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https://imgur.com/a/ZnVQ7j1