Check out the FAL Files referenced above. I believe most Imbel receivers are metric, but I've read that some inch versions may have been made (special order for Century?).
Also, I believe some have been altered to accempt inch magazines (which evidently makes them less reliable with the cheaper and more common metric mags). I understand that Century Arms assembled some rifles using a mix of metric and inch parts.
I'm certainly not an expert, but I wouldn't assume that an Imbel receiver always means 100% metric. I did, however, recently purchase a new Century STG-58 rifle built on an Imbel receiver. I'm very pleased. Metric mags fit perfectly. Seems well put together (despite Century's sometimes poor reputation). I paid $589.95 from AIM Surplus (plus FFL fee, tax, shipping, etc.). Also got some new metric mags from AIM for $7.95 each.