Yes counter shaft the shafts are the same dia but the Old style shaft for the cast iron case does not rotate. The 1/2 moon (woodruff) key on the end prevents that. I do not think they will interchange because of the length and the shaft is shorter on the OD trans to allow for the plug in the case at the end of the shaft and the OD shaft does not have the woodruff key slot.

The problem with the Aluminum case is that they used a floating counter shaft to make it quieter. Most new transmissions today do also but they either put a bushing or bearing in the case for support. The steel shaft rotating in the aluminum case leads to wallowed out holes in the aluminum case and the cluster gear begins to move. When the case wallows out and the cluster gear moves the trans get's noisy and the cluster gear fails.