many Late model 66's had a reverse and early 66's were commonly found in treadles, same with 15's. almost any machine with an external motor and belt drive can be treadles....there are some later machine with a built in Belt guard that extends to far down the front and prevents a treadle belt from being installed. I personally use a 306 and know of some319's in treadle stands ( not recommended to go buy because of obsolete needle issues )