I don't own one. But many years ago I operated a Merrow that was set to apply elastic to pull-on slacks. I don't recall the model number.

What color is the "head" (the actual machine itself)? black indicates a really old machine, after that, the factory colors tended to range from grey through a khaki color. Is the bench formica (what color?) or wooden? This will also provide clues as to the machine's age.

If you do a search on the web you can probably winnow down the models (the serial number will be on the head of your machine and will give a big clue) and locate a technical and operator's manual for it (you want BOTH!). If you have a good mechanic in your area (not the Sew 'n' Vac guy, probably), you can have him research it for you. Those manuals will give you the needle size and adjustments for timing and tension. This is important stuff to have on hand when you own an older machine.

Not knowing how old it is or its condition I would suggest you locate an industrial sewing machine shop and take it in for "going over". It will be money well spent, trust me.

Can you post a picture of it?