Hy Noonan, the 51-8091 is the correct type, a Hydrovac. 51-8021 is a newer type. There is an outfit in Ontario that rebuilds them, their name escapes me at the moment. When looking online there will be different measurements, they are the length of the main body, they vary by the numbers of wheel cylinders in your braking system. You will want one with the same body measurement as yours, hope that helps.
