"The IRS makes a notable observation in the preamble to the proposed regulations about the service provider's obligation to maintain documents, stating that taxpayers have a duty to maintain sufficient documents to establish the tax basis in property, and that this duty extends until the expiration of the period of limitations following the date of the disposition of the property (i.e., the taxable disposition of the property subject to the 83(b) election, which could occur years after the date of the 83(b) election itself). Current recordkeeping practice may not correspond to the IRS' view."