Year when Kawa changed inner tubes of front forks

Hello all! I'm looking for new tubes for the front forks of my 1974 KZ400 (part # 44013-049) and am having trouble finding new, oem ones (there's a single one on ebay for just under $90). Does anyone know the years that are compatible with this part #? Would it be too awful if I were to purchase one NOS and one used one? Unless I'm missing something, they appear to quite literally just be metal tubes, so it seems as though this is maybe one item I can buy used, no? (Yes, I need new seals, dust shields as well - those I'll definitely get new old stock) As ever, THANK YOU!!!!

Why are you looking to change them ? If corroded and you are struggling to find replacements you can have the originals re-chromed for approx $150 a pair. They are just metal tubes but are then "hard chromed" and precision ground to the correct size diameter for the seals and bushes

On Partzilla the tubes have the same part number from 74 to 77.  They changed in 78.  A different part number means they are not the same...possibly the threaded end is different.  

FWIW. regarding a 78 model. The KZ400-A1 has many of the exact same fork parts as the 74 KZ400. I didn't check everything, but the upper/long spring, top fork bolt & o-ring, bottom bolt, fork seals, & some other stuff that's kind of directly related to the inner cylinder are the same.  The 78 lower/short spring and fork cylinder have different part numbers, but those parts are the same as the 77 which had the same inner cylinder as the 74, which would tend to indicate the parts are interchangeable.
1974 >> www.partzilla.com/catalog/kawasaki/motor...k-74-75-kz400-kz400d
1977 >>  www.partzilla.com/catalog/kawasaki/motor...rk-76-77-kz400-d3-d4
1978 >>  www.partzilla.com/catalog/kawasaki/motor.../kz400-a2/front-fork

Part numbers.  In some cases Kawasaki would make a minor change to a part, and any change is reflected in a new part number; sometimes the new part fits exactly the same, but I can't say whether or not that applies here. I'd guess yes.

Because my mechanic says I need to - haha. Not sure as to the specific reason, I'll text him now, as spending $150 to get them re-chromed seems simpler and less expensive than sourcing them online

So I'm gonna make what I assume to be an educated guess and say - yes, it's the chrome (or rather, lack thereof). No?

Yes the chrome is past it's " use by date"   the below is a firm in California (which is where I believe you said you were)

www.ecgrinding.com/motorcycle-fork-repair.html