I would consider your handguards to be "tool room experiments", and not early M16A2 handguards, or even M16A1E1 handguards. The round handguards date back to the 1960's, and there was short and long versions, short version used on the XM177E1 and XM177E2, etc. These experimental handguards both long and short used a round handguard cap.
The "regular production" M16A1E1 had a triangle handguard cap, so that the handguards would be interchangeable with M16A1 handguards. As am sure you already noticed the rifle just below the one you mentioned is a M16A1E1 with triangle handguard cap. Here is a digital M16A1E1 picture I just happen to have on hand:
Don't have it with me, but there are a few pictures of the early experimental round handguards in TBR, one is in the late 60's section, and is just a picture with a short notation. There is also a write up about a 1970's Navy experiment with these handguards, complete with a few pictures.
The M16A1E1 was Coldblue's baby, and he could provide details in regards to the handguards used on those rifles.
And yes, we want pics!