Chris, I have done a complete (
AFAIK) list of paints for the 914, and that has taken some 20 years. I have been trying to work on the 911 as well because some of them cross over. Also, with the custom colors things can get interesting. 914's have two custom codes, 98 & 99, where 98 indicates an existing paint in the VW & Porsche systems while 99 is a true custom match to a customer supplied sample (that may be an existing paint in another manufacturers system). Your car may be the latter in which case you won't find a match in existing Porsche codes. The original owner that ordered the car would be the best source for help. Such a custom color puts a good premium on the car if it is desirable such as yours appears to be. It can be a curse if you don't know the correct paint code for repainting.
The part number is a regular part code in the paint section, however you had a 303 in the # and I cannot find that section, only a 903 section. Either that is a typo, or it is completely non-standard. My parts price manual is an 1986 version, so that may be part of the problem. It does list a lot of supercessions, so it should not be that bad, however it is 12 years later than your car. The other problem with the paint part numbers is that the description says 'laquer' and is no help at all. The 624 does seem to be in the ballpark for grey though, so that may be a clue. Has PCNA been any help?
Dave