that looks like a hodge podge of images you've stuck together, which doesn't really help. Almost looks like you're entering data in tables. What would be better is an explanation of what you have (form-wise, table-wise, etc.) and the process you're following rather than an explanation of what doesn't work. Things like "there is only 1 cylinder per asset" and "choosing an asset from combo on main form..." (as opposed to I'd like to enter (where? how?), "I only use macros/can use code" are important and make it easier and faster for us to help you.
It ought to be as simple as getting that 1 to 1 information (if that's what it is) using a combo AfterUpdate event if that's the input method. If you've been researching and can't find something that works, then there could be something about your setup that's an issue, because this is a commonly asked question. Hopefully you have not created multi value fields or lookup fields in your table(s). These can make such tasks all the more difficult.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.