If you mean what is the part number for the blank eprom used to hold the game software then yes that is listed in the WMS-40X service manual. The eproms used for the game software are 20C010, 27C040 and 27C080 (or the equivalent). The 27C080 is not listed in the chart below but some of the WMS-40X games that have a lot of sound effects require the 27C080 because it is a larger storage size chip, holds more software data than the other eproms. The chips you'd be concerned with would be game & data chips XU2, XU3, and sound chips XU17, XU18, XU30 and XU31. On some games they didn't use all 4 sound chips, so Reel 'Em In might only have 2 or 3 sound chips.
(click to enlarge)
If you mean you want part numbers for the programmed chips then there isn't a source or list of the game software by part number that I have seen published but it could be out there and I just haven't seen it. Some of the other folks here that are familiar with the WMS-40x games may know more about it. If I needed software for the game I'd have to just ask if anyone has game software for Reel 'Em In, tell them if I needed 2 coin or 3 coin or 5 coin. When they responded I'd then ask about the different player payback percentage versions they might have to choose from.
When you see photos of the original WMS-40x game software chips there are labels on them with the Williams part numbers so I know they exist. Hobbyists that focus on the WMS-40x machines may have made a list of the game software with the part numbers, playback percentages, etc that they have written down over time. In the WMS-40x service manual parts section it just says to order the game software chips by game name (see the chart above, right hand column). Probably when a casino ordered the game software they'd call up their Williams rep and tell them the game software they needed, then he'd ask which version according to payback percentage and max coin that the casino wanted. There probably was a list somewhere in a document from Williams with all the WMS-40x game software they released, but I haven't run across it.
Here's an example of some Williams software chips with original labels. It isn't a very clear photo, only one I could find right now (these chips might be for the WMS-550 model, which is similar in some ways to the WMS-40x models):