I have the fuel filter off my '89 GT, it's a mahle KL 21. I'm comparing it to one I bought years ago for a '78 I used to have, it's a purolator GF-142 and the box sayes it equates to a 928.110.147.04 part #. I know the '78/'79 928 used a different filter but these two filters both have the same fittings on each end and the same overall height and width. Only difference is the purolator housing is about 1/2" longer with less shoulder to it. Looks to me like I should be able to use it on the '89. What gives, how are the early and later filters different, flow or something?

 

BTDT. I have an 83 with CIS, and was sold a KL21, which was a good inch shorter,and with a different inlet configuration, the outlet end hangs out of the mounting bracket a lot. Went to see my wrench, who says yes, thats how they come now, and anyway with CIS you should have KL22 to handle the higher system pressure of CIS (75 vs 37 psi). I suspect your 78/9 filter is an early version of KL22, so it should be fine on a 37psi LH system.
jp 83 Euro S AT 49k