Just to be sure, can you get the model number or serial number of the mixer? Then you can call KitchenAid and verify.
I found this on their website
www.kitchenaid.com
KitchenAid customers can call the Customer Satisfaction Center at 1-800-541-6390
Also this on their FAQ Page for Stand Mixers:
Will all the attachments fit on my stand mixer?
Yes, all the attachments that work off the attachment hub work on both 4-1/2 and 5-quart stand mixers. The flat beater, wire whip and dough hook, which come with each mixer, are not interchangeable. The dough hook, wire whip and flat beater for the 4-1/2-quart mixer do not work with the 5-quart model. Similarly, the dough hook, wire whip and flat beater for the 5-quart stand mixer do not work with the 4-1/2-quart model. Always turn the stand mixer off before putting on and taking off attachments. The water jacket is the only attachment that is exclusive to the 5-quart model. It will not work with the 4-1/2-quart stand mixer. Accessories are not recommended for use on the commercial KSMC50SWH
I bought my 5-quart heavy duty KitchenAid about 8 years ago and I just love it.
Good luck.