The ones I see on eBay (etc) don't give any sizes (i.e. weight of shot powder).

Look for a Herters Model 72 shotgun press. Look at the powder/shot measure that sits on top. The measure can be mounted on you bench on a standard powder measure base and is almost infinitely adjustable by inserting or removing washers from each drum. I have seen those measures alone offered on ebay. This is the best solution you're likely to find for an old shotgun press.

Found one, thanks.