Also note Lippert makes a 1621 AND a 1621HD pin box and they are different. Like Dave, I too have a 1621HD pin box, but it's on a 3670BH, it's a popular box. On the back of the pin box frame (that's the 12" wide U-channel that the pin box bolts to) should be a label to confirm the pin box frame. I'm not an expert but one could deduce a 1621HD pin box frame utilizes a 1621HD pin box. The Reese company should be able to tell you their xyz pin box is a drop in for the Lippert 1621HD pin box. Before you buy though, try to get in writing that the Reese's pin box has the exact geometry the 1621HD does. I had a problem because the "Direct replacement" Trailair pin box (L15) did not work with my new body style Chevy/GM, you have a classic body style and would not have my problem. Tom