Looking at your manual. That pot is at VR351 on your Vertical/Horizontal monitor board. It shows that pot to be a 50k ohm pot. There are several types of leg spacings on pots, so that would be the only trick.
Here is a page full of pots, that you can order from Bob Roberts. You can look for the 50k pot that best resembles the leg spacing of your original.
http://www.therealbobroberts.net/pots.html
By the way that other pot for your vertical size is a 20k pot, in case that one breaks. If you decide to order from Bob I'd order and replace them both. Bob runs that site, and if you email him with an order, you can give him for monitor model #, and tell him your looking for the 50k hor. hold pot for it, and ask him which recommends. He knows his stuff, and he's sure to help answer your question.
He cross ships priority mail, so most shipping starts at $7. So it's best to get several parts you need to make shipping on them reasonable. If I order on a weekday I usually get my order in 3-4 days from placing it. The only strange thing is that he accepts only money orders or checks. Since I don't use checks I get his address, go down to the post office the next day, pay for a stamp and the money order for the total amount with my debit card. It's simple and easy, and you can get your stamp and money order in one place without using cash.